diff --git a/src/canBe.ts b/src/canBe.ts
index 0e8d7b05..050c9e55 100644
--- a/src/canBe.ts
+++ b/src/canBe.ts
@@ -1,9 +1,5 @@
 import { hasValueInReadOnlyStringList } from './_internal';
-import {
-  HANGUL_CHARACTERS_BY_FIRST_INDEX,
-  HANGUL_CHARACTERS_BY_LAST_INDEX,
-  HANGUL_CHARACTERS_BY_MIDDLE_INDEX,
-} from './constants';
+import { CHOSEONGS, JONGSEONGS, JUNSEONGS } from './constants';
 
 /**
  * @name canBeChoseong
@@ -22,8 +18,8 @@ import {
  * canBeChoseong('ㅏ') // false
  * canBeChoseong('가') // false
  */
-export function canBeChoseong(character: string): character is (typeof HANGUL_CHARACTERS_BY_FIRST_INDEX)[number] {
-  return hasValueInReadOnlyStringList(HANGUL_CHARACTERS_BY_FIRST_INDEX, character);
+export function canBeChoseong(character: string): character is (typeof CHOSEONGS)[number] {
+  return hasValueInReadOnlyStringList(CHOSEONGS, character);
 }
 
 /**
@@ -44,8 +40,8 @@ export function canBeChoseong(character: string): character is (typeof HANGUL_CH
  * canBeJungseong('ㄱㅅ') // false
  * canBeJungseong('가') // false
  */
-export function canBeJungseong(character: string): character is (typeof HANGUL_CHARACTERS_BY_MIDDLE_INDEX)[number] {
-  return hasValueInReadOnlyStringList(HANGUL_CHARACTERS_BY_MIDDLE_INDEX, character);
+export function canBeJungseong(character: string): character is (typeof JUNSEONGS)[number] {
+  return hasValueInReadOnlyStringList(JUNSEONGS, character);
 }
 
 /**
@@ -66,6 +62,6 @@ export function canBeJungseong(character: string): character is (typeof HANGUL_C
  * canBeJongseong('ㅏ') // false
  * canBeJongseong('ㅗㅏ') // false
  */
-export function canBeJongseong(character: string): character is (typeof HANGUL_CHARACTERS_BY_LAST_INDEX)[number] {
-  return hasValueInReadOnlyStringList(HANGUL_CHARACTERS_BY_LAST_INDEX, character);
+export function canBeJongseong(character: string): character is (typeof JONGSEONGS)[number] {
+  return hasValueInReadOnlyStringList(JONGSEONGS, character);
 }
diff --git a/src/combineCharacter.ts b/src/combineCharacter.ts
index 762d8145..75647f38 100644
--- a/src/combineCharacter.ts
+++ b/src/combineCharacter.ts
@@ -2,9 +2,9 @@ import { canBeChoseong, canBeJongseong, canBeJungseong } from './canBe';
 import {
   COMPLETE_HANGUL_START_CHARCODE,
   DISASSEMBLED_VOWELS_BY_VOWEL,
-  HANGUL_CHARACTERS_BY_FIRST_INDEX,
-  HANGUL_CHARACTERS_BY_LAST_INDEX,
-  HANGUL_CHARACTERS_BY_MIDDLE_INDEX,
+  CHOSEONGS,
+  JONGSEONGS,
+  JUNSEONGS,
 } from './constants';
 
 /**
@@ -14,38 +14,33 @@ import {
  * ```typescript
  * combineCharacter(
  *   // 초성
- *   firstCharacter: string
+ *   choseong: string
  *   // 중성
- *   middleCharacter: string
+ *   jungseong: string
  *   // 종성
- *   lastCharacter: string
+ *   jongseong: string
  * ): string
  * ```
  * @example
  * combineCharacter('ㄱ', 'ㅏ', 'ㅂㅅ') // '값'
  * combineCharacter('ㅌ', 'ㅗ') // '토'
  */
-export function combineCharacter(firstCharacter: string, middleCharacter: string, lastCharacter = '') {
-  if (
-    canBeChoseong(firstCharacter) === false ||
-    canBeJungseong(middleCharacter) === false ||
-    canBeJongseong(lastCharacter) === false
-  ) {
-    throw new Error(`Invalid hangul Characters: ${firstCharacter}, ${middleCharacter}, ${lastCharacter}`);
+export function combineCharacter(choseong: string, jungseong: string, jongseong = '') {
+  if (canBeChoseong(choseong) === false || canBeJungseong(jungseong) === false || canBeJongseong(jongseong) === false) {
+    throw new Error(`Invalid hangul Characters: ${choseong}, ${jungseong}, ${jongseong}`);
   }
 
-  const numOfMiddleCharacters = HANGUL_CHARACTERS_BY_MIDDLE_INDEX.length;
-  const numOfLastCharacters = HANGUL_CHARACTERS_BY_LAST_INDEX.length;
+  const numOfJungseongs = JUNSEONGS.length;
+  const numOfJongseongs = JONGSEONGS.length;
 
-  const firstCharacterIndex = HANGUL_CHARACTERS_BY_FIRST_INDEX.indexOf(firstCharacter);
-  const middleCharacterIndex = HANGUL_CHARACTERS_BY_MIDDLE_INDEX.indexOf(middleCharacter);
-  const lastCharacterIndex = HANGUL_CHARACTERS_BY_LAST_INDEX.indexOf(lastCharacter);
+  const choseongIndex = CHOSEONGS.indexOf(choseong as (typeof CHOSEONGS)[number]);
+  const jungseongIndex = JUNSEONGS.indexOf(jungseong as (typeof JUNSEONGS)[number]);
+  const jongseongIndex = JONGSEONGS.indexOf(jongseong as (typeof JONGSEONGS)[number]);
 
-  const firstIndexOfTargetConsonant = firstCharacterIndex * numOfMiddleCharacters * numOfLastCharacters;
-  const firstIndexOfTargetVowel = middleCharacterIndex * numOfLastCharacters;
+  const choseongOfTargetConsonant = choseongIndex * numOfJungseongs * numOfJongseongs;
+  const choseongOfTargetVowel = jungseongIndex * numOfJongseongs;
 
-  const unicode =
-    COMPLETE_HANGUL_START_CHARCODE + firstIndexOfTargetConsonant + firstIndexOfTargetVowel + lastCharacterIndex;
+  const unicode = COMPLETE_HANGUL_START_CHARCODE + choseongOfTargetConsonant + choseongOfTargetVowel + jongseongIndex;
 
   return String.fromCharCode(unicode);
 }
@@ -60,10 +55,10 @@ export function combineCharacter(firstCharacter: string, middleCharacter: string
  * combineLastHangulCharacter('ㄱ') // '각'
  */
 export const curriedCombineCharacter =
-  (firstCharacter: string) =>
-  (middleCharacter: string) =>
-  (lastCharacter = '') =>
-    combineCharacter(firstCharacter, middleCharacter, lastCharacter);
+  (choseong: string) =>
+  (jungseong: string) =>
+  (jongseong = '') =>
+    combineCharacter(choseong, jungseong, jongseong);
 
 /**
  * @name combineVowels
diff --git a/src/constants.ts b/src/constants.ts
index 20fa1a03..e9704ff9 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -79,7 +79,7 @@ export const DISASSEMBLED_VOWELS_BY_VOWEL = {
 /**
  * 초성으로 올 수 있는 한글 글자
  */
-export const HANGUL_CHARACTERS_BY_FIRST_INDEX = [
+export const CHOSEONGS = [
   'ㄱ',
   'ㄲ',
   'ㄴ',
@@ -104,12 +104,12 @@ export const HANGUL_CHARACTERS_BY_FIRST_INDEX = [
 /**
  * 중성으로 올 수 있는 한글 글자
  */
-export const HANGUL_CHARACTERS_BY_MIDDLE_INDEX = Object.values(DISASSEMBLED_VOWELS_BY_VOWEL);
+export const JUNSEONGS = Object.values(DISASSEMBLED_VOWELS_BY_VOWEL);
 
 /**
  * 종성으로 올 수 있는 한글 글자
  */
-export const HANGUL_CHARACTERS_BY_LAST_INDEX = (
+export const JONGSEONGS = (
   [
     '',
     'ㄱ',
diff --git a/src/disassemble.ts b/src/disassemble.ts
index f16e0b42..4f0b9b3f 100644
--- a/src/disassemble.ts
+++ b/src/disassemble.ts
@@ -16,7 +16,11 @@ export function disassembleToGroups(str: string) {
     const disassembledComplete = disassembleCompleteCharacter(letter);
 
     if (disassembledComplete != null) {
-      result.push([...disassembledComplete.first, ...disassembledComplete.middle, ...disassembledComplete.last]);
+      result.push([
+        ...disassembledComplete.choseong,
+        ...disassembledComplete.jungseong,
+        ...disassembledComplete.jongseong,
+      ]);
       continue;
     }
 
diff --git a/src/disassembleCompleteCharacter.spec.ts b/src/disassembleCompleteCharacter.spec.ts
index 716e3a25..62be5230 100644
--- a/src/disassembleCompleteCharacter.spec.ts
+++ b/src/disassembleCompleteCharacter.spec.ts
@@ -3,33 +3,33 @@ import { disassembleCompleteCharacter } from './disassembleCompleteCharacter';
 describe('disassembleCompleteCharacter', () => {
   it('값', () => {
     expect(disassembleCompleteCharacter('값')).toEqual({
-      first: 'ㄱ',
-      middle: 'ㅏ',
-      last: 'ㅂㅅ',
+      choseong: 'ㄱ',
+      jungseong: 'ㅏ',
+      jongseong: 'ㅂㅅ',
     });
   });
 
   it('리', () => {
     expect(disassembleCompleteCharacter('리')).toEqual({
-      first: 'ㄹ',
-      middle: 'ㅣ',
-      last: '',
+      choseong: 'ㄹ',
+      jungseong: 'ㅣ',
+      jongseong: '',
     });
   });
 
   it('빚', () => {
     expect(disassembleCompleteCharacter('빚')).toEqual({
-      first: 'ㅂ',
-      middle: 'ㅣ',
-      last: 'ㅈ',
+      choseong: 'ㅂ',
+      jungseong: 'ㅣ',
+      jongseong: 'ㅈ',
     });
   });
 
   it('박', () => {
     expect(disassembleCompleteCharacter('박')).toEqual({
-      first: 'ㅂ',
-      middle: 'ㅏ',
-      last: 'ㄱ',
+      choseong: 'ㅂ',
+      jungseong: 'ㅏ',
+      jongseong: 'ㄱ',
     });
   });
 
diff --git a/src/disassembleCompleteCharacter.ts b/src/disassembleCompleteCharacter.ts
index 43e54900..4a4337cb 100644
--- a/src/disassembleCompleteCharacter.ts
+++ b/src/disassembleCompleteCharacter.ts
@@ -1,17 +1,17 @@
 import {
   COMPLETE_HANGUL_END_CHARCODE,
   COMPLETE_HANGUL_START_CHARCODE,
-  HANGUL_CHARACTERS_BY_FIRST_INDEX,
-  HANGUL_CHARACTERS_BY_LAST_INDEX,
-  HANGUL_CHARACTERS_BY_MIDDLE_INDEX,
+  CHOSEONGS,
+  JONGSEONGS,
+  JUNSEONGS,
   NUMBER_OF_JONGSEONG,
   NUMBER_OF_JUNGSEONG,
 } from './constants';
 
 interface ReturnTypeDisassembleCompleteCharacter {
-  first: (typeof HANGUL_CHARACTERS_BY_FIRST_INDEX)[number];
-  middle: (typeof HANGUL_CHARACTERS_BY_MIDDLE_INDEX)[number];
-  last: (typeof HANGUL_CHARACTERS_BY_LAST_INDEX)[number];
+  choseong: (typeof CHOSEONGS)[number];
+  jungseong: (typeof JUNSEONGS)[number];
+  jongseong: (typeof JONGSEONGS)[number];
 }
 
 /**
@@ -22,10 +22,10 @@ interface ReturnTypeDisassembleCompleteCharacter {
  * @param {string} letter 분리하고자 하는 완전한 한글 문자열
  *
  * @example
- * disassembleCompleteCharacter('값') // { first: 'ㄱ', middle: 'ㅏ', last: 'ㅂㅅ' }
- * disassembleCompleteCharacter('리') // { first: 'ㄹ', middle: 'ㅣ', last: '' }
- * disassembleCompleteCharacter('빚') // { first: 'ㅂ', middle: 'ㅣ', last: 'ㅈ' }
- * disassembleCompleteCharacter('박') // { first: 'ㅂ', middle: 'ㅏ', last: 'ㄱ' }
+ * disassembleCompleteCharacter('값') // { choseong: 'ㄱ', jungseong: 'ㅏ', jongseong: 'ㅂㅅ' }
+ * disassembleCompleteCharacter('리') // { choseong: 'ㄹ', jungseong: 'ㅣ', jongseong: '' }
+ * disassembleCompleteCharacter('빚') // { choseong: 'ㅂ', jungseong: 'ㅣ', jongseong: 'ㅈ' }
+ * disassembleCompleteCharacter('박') // { choseong: 'ㅂ', jungseong: 'ㅏ', jongseong: 'ㄱ' }
  */
 
 export function disassembleCompleteCharacter(letter: string): ReturnTypeDisassembleCompleteCharacter | undefined {
@@ -39,13 +39,13 @@ export function disassembleCompleteCharacter(letter: string): ReturnTypeDisassem
 
   const hangulCode = charCode - COMPLETE_HANGUL_START_CHARCODE;
 
-  const lastIndex = hangulCode % NUMBER_OF_JONGSEONG;
-  const middleIndex = ((hangulCode - lastIndex) / NUMBER_OF_JONGSEONG) % NUMBER_OF_JUNGSEONG;
-  const firstIndex = Math.floor((hangulCode - lastIndex) / NUMBER_OF_JONGSEONG / NUMBER_OF_JUNGSEONG);
+  const jongseongIndex = hangulCode % NUMBER_OF_JONGSEONG;
+  const jungseongIndex = ((hangulCode - jongseongIndex) / NUMBER_OF_JONGSEONG) % NUMBER_OF_JUNGSEONG;
+  const choseongIndex = Math.floor((hangulCode - jongseongIndex) / NUMBER_OF_JONGSEONG / NUMBER_OF_JUNGSEONG);
 
   return {
-    first: HANGUL_CHARACTERS_BY_FIRST_INDEX[firstIndex],
-    middle: HANGUL_CHARACTERS_BY_MIDDLE_INDEX[middleIndex],
-    last: HANGUL_CHARACTERS_BY_LAST_INDEX[lastIndex],
+    choseong: CHOSEONGS[choseongIndex],
+    jungseong: JUNSEONGS[jungseongIndex],
+    jongseong: JONGSEONGS[jongseongIndex],
   } as const;
 }
diff --git a/src/disassembleCompleteHangulCharacter.spec.ts b/src/disassembleCompleteHangulCharacter.spec.ts
new file mode 100644
index 00000000..62be5230
--- /dev/null
+++ b/src/disassembleCompleteHangulCharacter.spec.ts
@@ -0,0 +1,40 @@
+import { disassembleCompleteCharacter } from './disassembleCompleteCharacter';
+
+describe('disassembleCompleteCharacter', () => {
+  it('값', () => {
+    expect(disassembleCompleteCharacter('값')).toEqual({
+      choseong: 'ㄱ',
+      jungseong: 'ㅏ',
+      jongseong: 'ㅂㅅ',
+    });
+  });
+
+  it('리', () => {
+    expect(disassembleCompleteCharacter('리')).toEqual({
+      choseong: 'ㄹ',
+      jungseong: 'ㅣ',
+      jongseong: '',
+    });
+  });
+
+  it('빚', () => {
+    expect(disassembleCompleteCharacter('빚')).toEqual({
+      choseong: 'ㅂ',
+      jungseong: 'ㅣ',
+      jongseong: 'ㅈ',
+    });
+  });
+
+  it('박', () => {
+    expect(disassembleCompleteCharacter('박')).toEqual({
+      choseong: 'ㅂ',
+      jungseong: 'ㅏ',
+      jongseong: 'ㄱ',
+    });
+  });
+
+  it('완전한 한글 문자열이 아니면 undefined를 반환해야 합니다.', () => {
+    expect(disassembleCompleteCharacter('ㄱ')).toBeUndefined;
+    expect(disassembleCompleteCharacter('ㅏ')).toBeUndefined;
+  });
+});
diff --git a/src/getChoseong.ts b/src/getChoseong.ts
index befd95f6..17cedf59 100644
--- a/src/getChoseong.ts
+++ b/src/getChoseong.ts
@@ -1,4 +1,4 @@
-import { HANGUL_CHARACTERS_BY_FIRST_INDEX, JASO_HANGUL_NFD } from './constants';
+import { CHOSEONGS, JASO_HANGUL_NFD } from './constants';
 
 /**
  * @name getChoseong
@@ -18,7 +18,7 @@ export function getChoseong(word: string) {
   return word
     .normalize('NFD')
     .replace(EXTRACT_CHOSEONG_REGEX, '') // NFD ㄱ-ㅎ, NFC ㄱ-ㅎ 외 문자 삭제
-    .replace(CHOOSE_NFD_CHOSEONG_REGEX, $0 => HANGUL_CHARACTERS_BY_FIRST_INDEX[$0.charCodeAt(0) - 0x1100]); // NFD to NFC
+    .replace(CHOOSE_NFD_CHOSEONG_REGEX, $0 => CHOSEONGS[$0.charCodeAt(0) - 0x1100]); // NFD to NFC
 }
 
 const EXTRACT_CHOSEONG_REGEX = new RegExp(
diff --git a/src/hasBatchim.ts b/src/hasBatchim.ts
index 9fcfc2a3..02e44c5f 100644
--- a/src/hasBatchim.ts
+++ b/src/hasBatchim.ts
@@ -1,7 +1,7 @@
 import {
   COMPLETE_HANGUL_END_CHARCODE,
   COMPLETE_HANGUL_START_CHARCODE,
-  HANGUL_CHARACTERS_BY_LAST_INDEX,
+  JONGSEONGS,
   NUMBER_OF_JONGSEONG,
 } from './constants';
 
@@ -50,11 +50,11 @@ export function hasBatchim(
   const batchimCode = (charCode - COMPLETE_HANGUL_START_CHARCODE) % NUMBER_OF_JONGSEONG;
 
   if (options?.only === 'single') {
-    return HANGUL_CHARACTERS_BY_LAST_INDEX[batchimCode].length === 1;
+    return JONGSEONGS[batchimCode].length === 1;
   }
 
   if (options?.only === 'double') {
-    return HANGUL_CHARACTERS_BY_LAST_INDEX[batchimCode].length === 2;
+    return JONGSEONGS[batchimCode].length === 2;
   }
 
   return batchimCode > 0;
diff --git a/src/josa.ts b/src/josa.ts
index 8f5de283..906c0b3e 100644
--- a/src/josa.ts
+++ b/src/josa.ts
@@ -37,7 +37,7 @@ function josaPicker(word: string, josa: JosaOption): string {
   const has받침 = hasBatchim(word);
   let index = has받침 ? 0 : 1;
 
-  const is종성ㄹ = disassembleCompleteCharacter(word[word.length - 1])?.last === 'ㄹ';
+  const is종성ㄹ = disassembleCompleteCharacter(word[word.length - 1])?.jongseong === 'ㄹ';
 
   const isCaseOf로 = has받침 && is종성ㄹ && 로_조사.includes(josa);
 
diff --git a/src/romanize.ts b/src/romanize.ts
index f2d5f7a7..4ff0c0fd 100644
--- a/src/romanize.ts
+++ b/src/romanize.ts
@@ -27,15 +27,16 @@ const romanizeSyllableHangul = (arrayHangul: string[], index: number): string =>
       ReturnType<typeof disassembleCompleteCharacter>
     >;
 
-    let choseong: (typeof 초성_알파벳_발음)[keyof typeof 초성_알파벳_발음] | 'l' = 초성_알파벳_발음[disassemble.first];
-    const jungseong = 중성_알파벳_발음[assemble([disassemble.middle]) as keyof typeof 중성_알파벳_발음];
-    const jongseong = 종성_알파벳_발음[disassemble.last as keyof typeof 종성_알파벳_발음];
+    let choseong: (typeof 초성_알파벳_발음)[keyof typeof 초성_알파벳_발음] | 'l' =
+      초성_알파벳_발음[disassemble.choseong];
+    const jungseong = 중성_알파벳_발음[assemble([disassemble.jungseong]) as keyof typeof 중성_알파벳_발음];
+    const jongseong = 종성_알파벳_발음[disassemble.jongseong as keyof typeof 종성_알파벳_발음];
 
     // 'ㄹ'은 모음 앞에서는 'r'로, 자음 앞이나 어말에서는 'l'로 적는다. 단, 'ㄹㄹ'은 'll'로 적는다. (ex.울릉, 대관령),
-    if (disassemble.first === 'ㄹ' && index > 0 && isHangulCharacter(arrayHangul[index - 1])) {
+    if (disassemble.choseong === 'ㄹ' && index > 0 && isHangulCharacter(arrayHangul[index - 1])) {
       const prevDisassemble = disassembleCompleteCharacter(arrayHangul[index - 1]);
 
-      if (prevDisassemble?.last === 'ㄹ') {
+      if (prevDisassemble?.jongseong === 'ㄹ') {
         choseong = 'l';
       }
     }
diff --git a/src/standardizePronunciation/index.ts b/src/standardizePronunciation/index.ts
index 3c4e20cf..7fd2ad36 100644
--- a/src/standardizePronunciation/index.ts
+++ b/src/standardizePronunciation/index.ts
@@ -146,7 +146,7 @@ function applyRules(params: ApplyParameters): {
 function assembleChangedHangul(disassembleHangul: Syllable[], notHangulPhrase: NotHangul[]): string {
   const changedSyllables = disassembleHangul
     .filter(isNotUndefined)
-    .map(syllable => combineCharacter(syllable.first, syllable.middle, syllable.last));
+    .map(syllable => combineCharacter(syllable.choseong, syllable.jungseong, syllable.jongseong));
 
   for (const { index, syllable } of notHangulPhrase) {
     changedSyllables.splice(index, 0, syllable);
diff --git a/src/standardizePronunciation/rules/rules.utils.ts b/src/standardizePronunciation/rules/rules.utils.ts
index fe0f330c..f857f8e8 100644
--- a/src/standardizePronunciation/rules/rules.utils.ts
+++ b/src/standardizePronunciation/rules/rules.utils.ts
@@ -1,5 +1,5 @@
 import { Syllable } from './rules.types';
 
-export function replace받침ㅎ(currentSyllable: Syllable): Syllable['last'] {
-  return currentSyllable.last.replace('ㅎ', '') as Syllable['last'];
+export function replace받침ㅎ(currentSyllable: Syllable): Syllable['jongseong'] {
+  return currentSyllable.jongseong.replace('ㅎ', '') as Syllable['jongseong'];
 }
diff --git a/src/standardizePronunciation/rules/transform12th.spec.ts b/src/standardizePronunciation/rules/transform12th.spec.ts
index 5a649def..e3159b53 100644
--- a/src/standardizePronunciation/rules/transform12th.spec.ts
+++ b/src/standardizePronunciation/rules/transform12th.spec.ts
@@ -9,14 +9,14 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㄴ',
-        middle: 'ㅗ',
-        last: '',
+        choseong: 'ㄴ',
+        jungseong: 'ㅗ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅋ',
-        middle: 'ㅗ',
-        last: '',
+        choseong: 'ㅋ',
+        jungseong: 'ㅗ',
+        jongseong: '',
       },
     });
   });
@@ -27,14 +27,14 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㄱ',
-        middle: 'ㅏ',
-        last: '',
+        choseong: 'ㄱ',
+        jungseong: 'ㅏ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅋ',
-        middle: 'ㅏ',
-        last: '',
+        choseong: 'ㅋ',
+        jungseong: 'ㅏ',
+        jongseong: '',
       },
     });
   });
@@ -45,14 +45,14 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㄷ',
-        middle: 'ㅏ',
-        last: '',
+        choseong: 'ㄷ',
+        jungseong: 'ㅏ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅆ',
-        middle: 'ㅗ',
-        last: '',
+        choseong: 'ㅆ',
+        jungseong: 'ㅗ',
+        jongseong: '',
       },
     });
   });
@@ -63,14 +63,14 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㄴ',
-        middle: 'ㅗ',
-        last: '',
+        choseong: 'ㄴ',
+        jungseong: 'ㅗ',
+        jongseong: '',
       },
       next: {
-        first: 'ㄴ',
-        middle: 'ㅡ',
-        last: 'ㄴ',
+        choseong: 'ㄴ',
+        jungseong: 'ㅡ',
+        jongseong: 'ㄴ',
       },
     });
   });
@@ -81,14 +81,14 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㅇ',
-        middle: 'ㅏ',
-        last: 'ㄴ',
+        choseong: 'ㅇ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄴ',
       },
       next: {
-        first: 'ㄴ',
-        middle: 'ㅔ',
-        last: '',
+        choseong: 'ㄴ',
+        jungseong: 'ㅔ',
+        jongseong: '',
       },
     });
   });
@@ -99,14 +99,14 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㄴ',
-        middle: 'ㅏ',
-        last: '',
+        choseong: 'ㄴ',
+        jungseong: 'ㅏ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅇ',
-        middle: 'ㅡ',
-        last: 'ㄴ',
+        choseong: 'ㅇ',
+        jungseong: 'ㅡ',
+        jongseong: 'ㄴ',
       },
     });
   });
@@ -117,9 +117,9 @@ describe('transform12th', () => {
 
     expect(transform12th(current, next)).toEqual({
       current: {
-        first: 'ㅁ',
-        middle: 'ㅏ',
-        last: 'ㄴ',
+        choseong: 'ㅁ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄴ',
       },
       next: null,
     });
diff --git a/src/standardizePronunciation/rules/transform12th.ts b/src/standardizePronunciation/rules/transform12th.ts
index 70a64cdc..77841f94 100644
--- a/src/standardizePronunciation/rules/transform12th.ts
+++ b/src/standardizePronunciation/rules/transform12th.ts
@@ -25,26 +25,26 @@ export function transform12th(currentSyllable: Syllable, nextSyllable: Nullable<
   let current = { ...currentSyllable };
   let next = nextSyllable ? { ...nextSyllable } : nextSyllable;
 
-  if (!current.last) {
+  if (!current.jongseong) {
     return {
       current,
       next,
     };
   }
 
-  if (arrayIncludes(발음변환_받침_ㅎ, current.last)) {
+  if (arrayIncludes(발음변환_받침_ㅎ, current.jongseong)) {
     if (next) {
-      ({ current, next } = handleNextFirstIsㄱㄷㅈㅅ(current, next));
-      ({ current, next } = handleNextFirstIsㄴ(current, next));
-      ({ current, next } = handleNextFirstIsㅇ(current, next));
+      ({ current, next } = handleNextChoseongIsㄱㄷㅈㅅ(current, next));
+      ({ current, next } = handleNextChoseongIsㄴ(current, next));
+      ({ current, next } = handleNextChoseongIsㅇ(current, next));
     }
 
     if (!next) {
-      ({ current } = handleCurrentLastIsㅇ(current));
+      ({ current } = handleCurrentJongseongIsㅇ(current));
     }
   }
 
-  ({ current, next } = handleNextFirstIsㅎ(current, next));
+  ({ current, next } = handleNextChoseongIsㅎ(current, next));
 
   return {
     current,
@@ -52,62 +52,62 @@ export function transform12th(currentSyllable: Syllable, nextSyllable: Nullable<
   };
 }
 
-function handleNextFirstIsㄱㄷㅈㅅ(current: Syllable, next: Syllable): ReturnSyllables {
+function handleNextChoseongIsㄱㄷㅈㅅ(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (arrayIncludes(['ㄱ', 'ㄷ', 'ㅈ', 'ㅅ'], updatedNext.first)) {
-    updatedNext.first = 발음변환_받침_ㅎ_발음[updatedNext.first as keyof typeof 발음변환_받침_ㅎ_발음];
-    updatedCurrent.last = replace받침ㅎ(updatedCurrent);
+  if (arrayIncludes(['ㄱ', 'ㄷ', 'ㅈ', 'ㅅ'], updatedNext.choseong)) {
+    updatedNext.choseong = 발음변환_받침_ㅎ_발음[updatedNext.choseong as keyof typeof 발음변환_받침_ㅎ_발음];
+    updatedCurrent.jongseong = replace받침ㅎ(updatedCurrent);
   }
 
   return { current: updatedCurrent, next: updatedNext };
 }
 
-function handleNextFirstIsㄴ(current: Syllable, next: Syllable): ReturnSyllables {
+function handleNextChoseongIsㄴ(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (updatedNext.first === 'ㄴ' && arrayIncludes(['ㄴㅎ', 'ㄹㅎ'], updatedCurrent.last)) {
-    updatedCurrent.last = replace받침ㅎ(updatedCurrent);
+  if (updatedNext.choseong === 'ㄴ' && arrayIncludes(['ㄴㅎ', 'ㄹㅎ'], updatedCurrent.jongseong)) {
+    updatedCurrent.jongseong = replace받침ㅎ(updatedCurrent);
   }
   return { current: updatedCurrent, next: updatedNext };
 }
 
-function handleNextFirstIsㅇ(current: Syllable, next: Syllable): ReturnSyllables {
+function handleNextChoseongIsㅇ(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (updatedNext.first === 음가가_없는_자음) {
-    if (arrayIncludes(['ㄴㅎ', 'ㄹㅎ'], updatedCurrent.last)) {
-      updatedCurrent.last = replace받침ㅎ(updatedCurrent);
+  if (updatedNext.choseong === 음가가_없는_자음) {
+    if (arrayIncludes(['ㄴㅎ', 'ㄹㅎ'], updatedCurrent.jongseong)) {
+      updatedCurrent.jongseong = replace받침ㅎ(updatedCurrent);
     } else {
-      updatedCurrent.last = '';
+      updatedCurrent.jongseong = '';
     }
   } else {
-    updatedCurrent.last = replace받침ㅎ(updatedCurrent);
+    updatedCurrent.jongseong = replace받침ㅎ(updatedCurrent);
   }
   return { current: updatedCurrent, next: updatedNext };
 }
 
-function handleCurrentLastIsㅇ(current: Syllable): Pick<ReturnSyllables, 'current'> {
+function handleCurrentJongseongIsㅇ(current: Syllable): Pick<ReturnSyllables, 'current'> {
   const updatedCurrent = { ...current };
 
-  updatedCurrent.last = replace받침ㅎ(updatedCurrent);
+  updatedCurrent.jongseong = replace받침ㅎ(updatedCurrent);
   return { current: updatedCurrent };
 }
 
-function handleNextFirstIsㅎ(current: Syllable, next: Nullable<Syllable>): NullableReturnSyllables {
+function handleNextChoseongIsㅎ(current: Syllable, next: Nullable<Syllable>): NullableReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = next ? { ...next } : next;
 
-  if (arrayIncludes(발음변환_첫소리_ㅎ, updatedCurrent.last) && arrayIncludes(['ㅎ'], updatedNext?.first)) {
-    updatedNext.first = 발음변환_첫소리_ㅎ_발음[updatedCurrent.last];
+  if (arrayIncludes(발음변환_첫소리_ㅎ, updatedCurrent.jongseong) && arrayIncludes(['ㅎ'], updatedNext?.choseong)) {
+    updatedNext.choseong = 발음변환_첫소리_ㅎ_발음[updatedCurrent.jongseong];
 
-    if (updatedCurrent.last.length === 1) {
-      updatedCurrent.last = '';
+    if (updatedCurrent.jongseong.length === 1) {
+      updatedCurrent.jongseong = '';
     } else {
-      updatedCurrent.last = updatedCurrent.last[0] as Syllable['last'];
+      updatedCurrent.jongseong = updatedCurrent.jongseong[0] as Syllable['jongseong'];
     }
   }
   return { current: updatedCurrent, next: updatedNext };
diff --git a/src/standardizePronunciation/rules/transform13And14th.spec.ts b/src/standardizePronunciation/rules/transform13And14th.spec.ts
index f09e636a..0c9aa1ec 100644
--- a/src/standardizePronunciation/rules/transform13And14th.spec.ts
+++ b/src/standardizePronunciation/rules/transform13And14th.spec.ts
@@ -9,14 +9,14 @@ describe('transform13And14th', () => {
 
     expect(transform13And14th(current, next)).toEqual({
       current: {
-        first: 'ㄲ',
-        middle: 'ㅏ',
-        last: '',
+        choseong: 'ㄲ',
+        jungseong: 'ㅏ',
+        jongseong: '',
       },
       next: {
-        first: 'ㄲ',
-        middle: 'ㅏ',
-        last: '',
+        choseong: 'ㄲ',
+        jungseong: 'ㅏ',
+        jongseong: '',
       },
     });
   });
@@ -27,14 +27,14 @@ describe('transform13And14th', () => {
 
     expect(transform13And14th(current, next)).toEqual({
       current: {
-        first: 'ㄷ',
-        middle: 'ㅏ',
-        last: 'ㄹ',
+        choseong: 'ㄷ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄹ',
       },
       next: {
-        first: 'ㄱ',
-        middle: 'ㅡ',
-        last: 'ㄹ',
+        choseong: 'ㄱ',
+        jungseong: 'ㅡ',
+        jongseong: 'ㄹ',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform13And14th.ts b/src/standardizePronunciation/rules/transform13And14th.ts
index faf02bd0..651dcea7 100644
--- a/src/standardizePronunciation/rules/transform13And14th.ts
+++ b/src/standardizePronunciation/rules/transform13And14th.ts
@@ -19,7 +19,7 @@ export function transform13And14th(currentSyllable: Syllable, nextSyllable: Syll
   let current = { ...currentSyllable };
   let next = { ...nextSyllable };
 
-  const 제13_14항주요조건 = current.last && next.first === 음가가_없는_자음;
+  const 제13_14항주요조건 = current.jongseong && next.choseong === 음가가_없는_자음;
 
   if (!제13_14항주요조건) {
     return {
@@ -38,24 +38,24 @@ export function transform13And14th(currentSyllable: Syllable, nextSyllable: Syll
 }
 
 function is홑받침(current: Syllable): boolean {
-  return current.last.length === 받침의길이['홀받침'];
+  return current.jongseong.length === 받침의길이['홀받침'];
 }
 
 function is쌍받침(current: Syllable): boolean {
-  return current.last.length === 받침의길이['쌍_겹받침'] && current.last[0] === current.last[1];
+  return current.jongseong.length === 받침의길이['쌍_겹받침'] && current.jongseong[0] === current.jongseong[1];
 }
 
 function is겹받침(current: Syllable): boolean {
-  return current.last.length === 받침의길이['쌍_겹받침'] && current.last[0] !== current.last[1];
+  return current.jongseong.length === 받침의길이['쌍_겹받침'] && current.jongseong[0] !== current.jongseong[1];
 }
 
 function handle홑받침or쌍받침(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (!arrayIncludes(['ㅇ', ''], updatedCurrent.last) && (is홑받침(updatedCurrent) || is쌍받침(updatedCurrent))) {
-    updatedNext.first = updatedCurrent.last;
-    updatedCurrent.last = '';
+  if (!arrayIncludes(['ㅇ', ''], updatedCurrent.jongseong) && (is홑받침(updatedCurrent) || is쌍받침(updatedCurrent))) {
+    updatedNext.choseong = updatedCurrent.jongseong;
+    updatedCurrent.jongseong = '';
   }
   return { current: updatedCurrent, next: updatedNext };
 }
@@ -65,12 +65,15 @@ function handle겹받침(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedNext = { ...next };
 
   if (is겹받침(updatedCurrent)) {
-    if (updatedCurrent.last[1] === 'ㅅ') {
-      updatedNext.first = 'ㅆ';
+    if (updatedCurrent.jongseong[1] === 'ㅅ') {
+      updatedNext.choseong = 'ㅆ';
     } else {
-      updatedNext.first = updatedCurrent.last[1] as Syllable['first'];
+      updatedNext.choseong = updatedCurrent.jongseong[1] as Syllable['choseong'];
     }
-    updatedCurrent.last = updatedCurrent.last.replace(updatedCurrent.last[1], '') as Syllable['last'];
+    updatedCurrent.jongseong = updatedCurrent.jongseong.replace(
+      updatedCurrent.jongseong[1],
+      ''
+    ) as Syllable['jongseong'];
   }
   return { current: updatedCurrent, next: updatedNext };
 }
diff --git a/src/standardizePronunciation/rules/transform16th.spec.ts b/src/standardizePronunciation/rules/transform16th.spec.ts
index ee38a500..2b30f921 100644
--- a/src/standardizePronunciation/rules/transform16th.spec.ts
+++ b/src/standardizePronunciation/rules/transform16th.spec.ts
@@ -18,14 +18,14 @@ describe('transform16th', () => {
       })
     ).toEqual({
       current: {
-        first: 'ㄱ',
-        middle: 'ㅡ',
-        last: '',
+        choseong: 'ㄱ',
+        jungseong: 'ㅡ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅅ',
-        middle: 'ㅣ',
-        last: '',
+        choseong: 'ㅅ',
+        jungseong: 'ㅣ',
+        jongseong: '',
       },
     });
   });
@@ -45,14 +45,14 @@ describe('transform16th', () => {
       })
     ).toEqual({
       current: {
-        first: 'ㅇ',
-        middle: 'ㅕ',
-        last: '',
+        choseong: 'ㅇ',
+        jungseong: 'ㅕ',
+        jongseong: '',
       },
       next: {
-        first: 'ㄱ',
-        middle: 'ㅣ',
-        last: '',
+        choseong: 'ㄱ',
+        jungseong: 'ㅣ',
+        jongseong: '',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform16th.ts b/src/standardizePronunciation/rules/transform16th.ts
index 1fcae938..5622b6a1 100644
--- a/src/standardizePronunciation/rules/transform16th.ts
+++ b/src/standardizePronunciation/rules/transform16th.ts
@@ -26,7 +26,7 @@ export function transform16th({ currentSyllable, phrase, index, nextSyllable }:
   let current = { ...currentSyllable };
   let next = { ...nextSyllable };
 
-  const 제16항주요조건 = current.last && next.first === 음가가_없는_자음;
+  const 제16항주요조건 = current.jongseong && next.choseong === 음가가_없는_자음;
 
   if (!제16항주요조건) {
     return {
@@ -57,10 +57,10 @@ function handleSpecialHangulCharacters({
   const updatedNext = { ...next };
 
   if (arrayIncludes(특별한_한글_자모, combinedSyllables)) {
-    const 다음_음절의_초성 = 특별한_한글_자모의_발음[updatedCurrent.last as keyof typeof 특별한_한글_자모의_발음];
+    const 다음_음절의_초성 = 특별한_한글_자모의_발음[updatedCurrent.jongseong as keyof typeof 특별한_한글_자모의_발음];
 
-    updatedCurrent.last = '';
-    updatedNext.first = 다음_음절의_초성;
+    updatedCurrent.jongseong = '';
+    updatedNext.choseong = 다음_음절의_초성;
   }
   return { current: updatedCurrent, next: updatedNext };
 }
@@ -76,10 +76,10 @@ function handleHangulCharacters({
   const updatedNext = { ...next };
 
   if (arrayIncludes(한글_자모, combinedSyllables)) {
-    updatedNext.first = updatedCurrent.last as typeof updatedNext.first;
+    updatedNext.choseong = updatedCurrent.jongseong as typeof updatedNext.choseong;
 
-    if (updatedCurrent.last !== 'ㅇ') {
-      updatedCurrent.last = '';
+    if (updatedCurrent.jongseong !== 'ㅇ') {
+      updatedCurrent.jongseong = '';
     }
   }
   return { current: updatedCurrent, next: updatedNext };
diff --git a/src/standardizePronunciation/rules/transform17th.spec.ts b/src/standardizePronunciation/rules/transform17th.spec.ts
index a371d420..0aead226 100644
--- a/src/standardizePronunciation/rules/transform17th.spec.ts
+++ b/src/standardizePronunciation/rules/transform17th.spec.ts
@@ -9,14 +9,14 @@ describe('transform17th', () => {
 
     expect(transform17th(current, next)).toEqual({
       current: {
-        first: 'ㄱ',
-        middle: 'ㅜ',
-        last: '',
+        choseong: 'ㄱ',
+        jungseong: 'ㅜ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅈ',
-        middle: 'ㅣ',
-        last: '',
+        choseong: 'ㅈ',
+        jungseong: 'ㅣ',
+        jongseong: '',
       },
     });
   });
@@ -27,14 +27,14 @@ describe('transform17th', () => {
 
     expect(transform17th(current, next)).toEqual({
       current: {
-        first: 'ㄱ',
-        middle: 'ㅜ',
-        last: '',
+        choseong: 'ㄱ',
+        jungseong: 'ㅜ',
+        jongseong: '',
       },
       next: {
-        first: 'ㅊ',
-        middle: 'ㅣ',
-        last: '',
+        choseong: 'ㅊ',
+        jungseong: 'ㅣ',
+        jongseong: '',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform17th.ts b/src/standardizePronunciation/rules/transform17th.ts
index 094a714c..a98f9a94 100644
--- a/src/standardizePronunciation/rules/transform17th.ts
+++ b/src/standardizePronunciation/rules/transform17th.ts
@@ -14,7 +14,7 @@ export function transform17th(currentSyllable: Syllable, nextSyllable: Syllable)
   let current = { ...currentSyllable };
   let next = { ...nextSyllable };
 
-  const 제17항주요조건 = next.middle === 'ㅣ';
+  const 제17항주요조건 = next.jungseong === 'ㅣ';
 
   if (!제17항주요조건) {
     return {
@@ -23,8 +23,8 @@ export function transform17th(currentSyllable: Syllable, nextSyllable: Syllable)
     };
   }
 
-  ({ current, next } = handleFirstIsㅇ(current, next));
-  ({ current, next } = handleFirstIsㅎAndㄷ(current, next));
+  ({ current, next } = handleChoseongIsㅇ(current, next));
+  ({ current, next } = handleChoseongIsㅎAndㄷ(current, next));
 
   return {
     current,
@@ -32,24 +32,24 @@ export function transform17th(currentSyllable: Syllable, nextSyllable: Syllable)
   };
 }
 
-function handleFirstIsㅇ(current: Syllable, next: Syllable): ReturnSyllables {
+function handleChoseongIsㅇ(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (updatedNext.first === 'ㅇ' && hasProperty(음의_동화_받침, updatedCurrent.last)) {
-    updatedNext.first = 음의_동화_받침[updatedCurrent.last];
-    updatedCurrent.last = updatedCurrent.last === 'ㄹㅌ' ? 'ㄹ' : '';
+  if (updatedNext.choseong === 'ㅇ' && hasProperty(음의_동화_받침, updatedCurrent.jongseong)) {
+    updatedNext.choseong = 음의_동화_받침[updatedCurrent.jongseong];
+    updatedCurrent.jongseong = updatedCurrent.jongseong === 'ㄹㅌ' ? 'ㄹ' : '';
   }
   return { current: updatedCurrent, next: updatedNext };
 }
 
-function handleFirstIsㅎAndㄷ(current: Syllable, next: Syllable): ReturnSyllables {
+function handleChoseongIsㅎAndㄷ(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (updatedNext.first === 'ㅎ' && updatedCurrent.last === 'ㄷ') {
-    updatedNext.first = 'ㅊ';
-    updatedCurrent.last = '';
+  if (updatedNext.choseong === 'ㅎ' && updatedCurrent.jongseong === 'ㄷ') {
+    updatedNext.choseong = 'ㅊ';
+    updatedCurrent.jongseong = '';
   }
   return { current: updatedCurrent, next: updatedNext };
 }
diff --git a/src/standardizePronunciation/rules/transform18th.spec.ts b/src/standardizePronunciation/rules/transform18th.spec.ts
index c8f0da0d..f2193cbc 100644
--- a/src/standardizePronunciation/rules/transform18th.spec.ts
+++ b/src/standardizePronunciation/rules/transform18th.spec.ts
@@ -9,9 +9,9 @@ describe('transform18th', () => {
 
     expect(transform18th(current, next)).toEqual({
       current: {
-        first: 'ㅁ',
-        middle: 'ㅓ',
-        last: 'ㅇ',
+        choseong: 'ㅁ',
+        jungseong: 'ㅓ',
+        jongseong: 'ㅇ',
       },
     });
   });
@@ -22,9 +22,9 @@ describe('transform18th', () => {
 
     expect(transform18th(current, next)).toEqual({
       current: {
-        first: 'ㄷ',
-        middle: 'ㅏ',
-        last: 'ㄴ',
+        choseong: 'ㄷ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄴ',
       },
     });
   });
@@ -35,9 +35,9 @@ describe('transform18th', () => {
 
     expect(transform18th(current, next)).toEqual({
       current: {
-        first: 'ㅈ',
-        middle: 'ㅏ',
-        last: 'ㅁ',
+        choseong: 'ㅈ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㅁ',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform18th.ts b/src/standardizePronunciation/rules/transform18th.ts
index 1f695105..4d806e73 100644
--- a/src/standardizePronunciation/rules/transform18th.ts
+++ b/src/standardizePronunciation/rules/transform18th.ts
@@ -12,7 +12,7 @@ import { ReturnSyllables, Syllable } from './rules.types';
 export function transform18th(currentSyllable: Syllable, nextSyllable: Syllable): Pick<ReturnSyllables, 'current'> {
   const current = { ...currentSyllable };
 
-  const 제18항주요조건 = current.last && arrayIncludes(['ㄴ', 'ㅁ'], nextSyllable.first);
+  const 제18항주요조건 = current.jongseong && arrayIncludes(['ㄴ', 'ㅁ'], nextSyllable.choseong);
 
   if (!제18항주요조건) {
     return {
@@ -20,16 +20,16 @@ export function transform18th(currentSyllable: Syllable, nextSyllable: Syllable)
     };
   }
 
-  if (arrayIncludes(비음화_받침_ㅇ_변환, current.last)) {
-    current.last = 'ㅇ';
+  if (arrayIncludes(비음화_받침_ㅇ_변환, current.jongseong)) {
+    current.jongseong = 'ㅇ';
   }
 
-  if (arrayIncludes(비음화_받침_ㄴ_변환, current.last)) {
-    current.last = 'ㄴ';
+  if (arrayIncludes(비음화_받침_ㄴ_변환, current.jongseong)) {
+    current.jongseong = 'ㄴ';
   }
 
-  if (arrayIncludes(비음화_받침_ㅁ_변환, current.last)) {
-    current.last = 'ㅁ';
+  if (arrayIncludes(비음화_받침_ㅁ_변환, current.jongseong)) {
+    current.jongseong = 'ㅁ';
   }
 
   return {
diff --git a/src/standardizePronunciation/rules/transform19th.spec.ts b/src/standardizePronunciation/rules/transform19th.spec.ts
index f7858357..41d6db59 100644
--- a/src/standardizePronunciation/rules/transform19th.spec.ts
+++ b/src/standardizePronunciation/rules/transform19th.spec.ts
@@ -9,9 +9,9 @@ describe('transform19th', () => {
 
     expect(transform19th(current, next)).toEqual({
       next: {
-        first: 'ㄴ',
-        middle: 'ㅕ',
-        last: 'ㄱ',
+        choseong: 'ㄴ',
+        jungseong: 'ㅕ',
+        jongseong: 'ㄱ',
       },
     });
   });
@@ -22,9 +22,9 @@ describe('transform19th', () => {
 
     expect(transform19th(current, next)).toEqual({
       next: {
-        first: 'ㄴ',
-        middle: 'ㅗ',
-        last: 'ㄴ',
+        choseong: 'ㄴ',
+        jungseong: 'ㅗ',
+        jongseong: 'ㄴ',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform19th.ts b/src/standardizePronunciation/rules/transform19th.ts
index 2a4b091e..6406705a 100644
--- a/src/standardizePronunciation/rules/transform19th.ts
+++ b/src/standardizePronunciation/rules/transform19th.ts
@@ -11,10 +11,10 @@ import { ReturnSyllables, Syllable } from './rules.types';
  */
 export function transform19th(currentSyllable: Syllable, nextSyllable: Syllable): Pick<ReturnSyllables, 'next'> {
   const next = { ...nextSyllable };
-  const 제19항조건 = arrayIncludes(자음동화_받침_ㄴ_변환, currentSyllable.last) && next.first === 'ㄹ';
+  const 제19항조건 = arrayIncludes(자음동화_받침_ㄴ_변환, currentSyllable.jongseong) && next.choseong === 'ㄹ';
 
   if (제19항조건) {
-    next.first = 'ㄴ';
+    next.choseong = 'ㄴ';
   }
 
   return { next };
diff --git a/src/standardizePronunciation/rules/transform20th.spec.ts b/src/standardizePronunciation/rules/transform20th.spec.ts
index 9efd4335..b1ffcd19 100644
--- a/src/standardizePronunciation/rules/transform20th.spec.ts
+++ b/src/standardizePronunciation/rules/transform20th.spec.ts
@@ -9,14 +9,14 @@ describe('transform20th', () => {
 
     expect(transform20th(current, next)).toEqual({
       current: {
-        first: 'ㄴ',
-        middle: 'ㅏ',
-        last: 'ㄹ',
+        choseong: 'ㄴ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄹ',
       },
       next: {
-        first: 'ㄹ',
-        middle: 'ㅗ',
-        last: '',
+        choseong: 'ㄹ',
+        jungseong: 'ㅗ',
+        jongseong: '',
       },
     });
   });
@@ -27,14 +27,14 @@ describe('transform20th', () => {
 
     expect(transform20th(current, next)).toEqual({
       current: {
-        first: 'ㄷ',
-        middle: 'ㅏ',
-        last: 'ㄹㅎ',
+        choseong: 'ㄷ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄹㅎ',
       },
       next: {
-        first: 'ㄹ',
-        middle: 'ㅡ',
-        last: 'ㄴ',
+        choseong: 'ㄹ',
+        jungseong: 'ㅡ',
+        jongseong: 'ㄴ',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform20th.ts b/src/standardizePronunciation/rules/transform20th.ts
index ef62bae5..970c56d2 100644
--- a/src/standardizePronunciation/rules/transform20th.ts
+++ b/src/standardizePronunciation/rules/transform20th.ts
@@ -24,8 +24,8 @@ export function transform20th(currentSyllable: Syllable, nextSyllable: Syllable)
 function applyMainCondition(current: Syllable, next: Syllable): Pick<ReturnSyllables, 'current'> {
   const updatedCurrent = { ...current };
 
-  if (updatedCurrent.last === 'ㄴ' && next.first === 'ㄹ') {
-    updatedCurrent.last = 'ㄹ';
+  if (updatedCurrent.jongseong === 'ㄴ' && next.choseong === 'ㄹ') {
+    updatedCurrent.jongseong = 'ㄹ';
   }
   return { current: updatedCurrent };
 }
@@ -33,8 +33,11 @@ function applyMainCondition(current: Syllable, next: Syllable): Pick<ReturnSylla
 function applySupplementaryCondition(current: Syllable, next: Syllable): Pick<ReturnSyllables, 'next'> {
   const updatedNext = { ...next };
 
-  if (updatedNext.first === 'ㄴ' && (current.last === 'ㄹ' || arrayIncludes(['ㄹㅎ', 'ㄹㅌ'], current.last))) {
-    updatedNext.first = 'ㄹ';
+  if (
+    updatedNext.choseong === 'ㄴ' &&
+    (current.jongseong === 'ㄹ' || arrayIncludes(['ㄹㅎ', 'ㄹㅌ'], current.jongseong))
+  ) {
+    updatedNext.choseong = 'ㄹ';
   }
   return { next: updatedNext };
 }
diff --git a/src/standardizePronunciation/rules/transform9And10And11th.spec.ts b/src/standardizePronunciation/rules/transform9And10And11th.spec.ts
index 6f10d4de..7278ea8b 100644
--- a/src/standardizePronunciation/rules/transform9And10And11th.spec.ts
+++ b/src/standardizePronunciation/rules/transform9And10And11th.spec.ts
@@ -9,9 +9,9 @@ describe('transform9And10And11th', () => {
 
     expect(transform9And10And11th(current, next)).toEqual({
       current: {
-        first: 'ㄷ',
-        middle: 'ㅏ',
-        last: 'ㄱ',
+        choseong: 'ㄷ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄱ',
       },
     });
   });
@@ -22,9 +22,9 @@ describe('transform9And10And11th', () => {
 
     expect(transform9And10And11th(current, next)).toEqual({
       current: {
-        first: 'ㅇ',
-        middle: 'ㅏ',
-        last: 'ㄴ',
+        choseong: 'ㅇ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄴ',
       },
     });
   });
@@ -35,9 +35,9 @@ describe('transform9And10And11th', () => {
 
     expect(transform9And10And11th(current, next)).toEqual({
       current: {
-        first: 'ㅎ',
-        middle: 'ㅡ',
-        last: 'ㄱ',
+        choseong: 'ㅎ',
+        jungseong: 'ㅡ',
+        jongseong: 'ㄱ',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transform9And10And11th.ts b/src/standardizePronunciation/rules/transform9And10And11th.ts
index 65643fbe..df77a217 100644
--- a/src/standardizePronunciation/rules/transform9And10And11th.ts
+++ b/src/standardizePronunciation/rules/transform9And10And11th.ts
@@ -16,13 +16,13 @@ export function transform9And10And11th(
 ): Pick<ReturnSyllables, 'current'> {
   const current = { ...currentSyllable };
 
-  const is어말 = current.last && !nextSyllable;
-  const is음가있는자음앞 = current.last && nextSyllable?.first !== 음가가_없는_자음;
+  const is어말 = current.jongseong && !nextSyllable;
+  const is음가있는자음앞 = current.jongseong && nextSyllable?.choseong !== 음가가_없는_자음;
 
-  const 제9_10_11항주요조건 = (is어말 || is음가있는자음앞) && hasProperty(받침_대표음_발음, current.last);
+  const 제9_10_11항주요조건 = (is어말 || is음가있는자음앞) && hasProperty(받침_대표음_발음, current.jongseong);
 
   if (제9_10_11항주요조건) {
-    current.last = 받침_대표음_발음[current.last as keyof typeof 받침_대표음_발음];
+    current.jongseong = 받침_대표음_발음[current.jongseong as keyof typeof 받침_대표음_발음];
   }
 
   return { current };
diff --git a/src/standardizePronunciation/rules/transformHardConversion.spec.ts b/src/standardizePronunciation/rules/transformHardConversion.spec.ts
index 39323f48..966d43fe 100644
--- a/src/standardizePronunciation/rules/transformHardConversion.spec.ts
+++ b/src/standardizePronunciation/rules/transformHardConversion.spec.ts
@@ -9,9 +9,9 @@ describe('transformHardConversion', () => {
 
     expect(transformHardConversion(current, next)).toEqual({
       next: {
-        first: 'ㅃ',
-        middle: 'ㅏ',
-        last: 'ㅂ',
+        choseong: 'ㅃ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㅂ',
       },
     });
   });
@@ -22,9 +22,9 @@ describe('transformHardConversion', () => {
 
     expect(transformHardConversion(current, next)).toEqual({
       next: {
-        first: 'ㄲ',
-        middle: 'ㅗ',
-        last: '',
+        choseong: 'ㄲ',
+        jungseong: 'ㅗ',
+        jongseong: '',
       },
     });
   });
@@ -35,9 +35,9 @@ describe('transformHardConversion', () => {
 
     expect(transformHardConversion(current, next)).toEqual({
       next: {
-        first: 'ㄲ',
-        middle: 'ㅔ',
-        last: '',
+        choseong: 'ㄲ',
+        jungseong: 'ㅔ',
+        jongseong: '',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transformHardConversion.ts b/src/standardizePronunciation/rules/transformHardConversion.ts
index 53275f1d..2b856f08 100644
--- a/src/standardizePronunciation/rules/transformHardConversion.ts
+++ b/src/standardizePronunciation/rules/transformHardConversion.ts
@@ -17,12 +17,12 @@ export function transformHardConversion(
 ): Pick<ReturnSyllables, 'next'> {
   const next = { ...nextSyllable };
 
-  if (hasProperty(된소리, next.first)) {
-    const 제23항조건 = arrayIncludes(된소리_받침, currentSyllable.last);
-    const 제24_25항조건 = arrayIncludes(어간_받침, currentSyllable.last) && next.first !== 'ㅂ';
+  if (hasProperty(된소리, next.choseong)) {
+    const 제23항조건 = arrayIncludes(된소리_받침, currentSyllable.jongseong);
+    const 제24_25항조건 = arrayIncludes(어간_받침, currentSyllable.jongseong) && next.choseong !== 'ㅂ';
 
     if (제23항조건 || 제24_25항조건) {
-      next.first = 된소리[next.first];
+      next.choseong = 된소리[next.choseong];
     }
   }
 
diff --git a/src/standardizePronunciation/rules/transformNLAssimilation.spec.ts b/src/standardizePronunciation/rules/transformNLAssimilation.spec.ts
index 5c183a80..b0467421 100644
--- a/src/standardizePronunciation/rules/transformNLAssimilation.spec.ts
+++ b/src/standardizePronunciation/rules/transformNLAssimilation.spec.ts
@@ -9,14 +9,14 @@ describe('transformNLAssimilation', () => {
 
     expect(transformNLAssimilation(current, next)).toEqual({
       current: {
-        first: 'ㅁ',
-        middle: 'ㅐ',
-        last: 'ㄴ',
+        choseong: 'ㅁ',
+        jungseong: 'ㅐ',
+        jongseong: 'ㄴ',
       },
       next: {
-        first: 'ㄴ',
-        middle: 'ㅣ',
-        last: 'ㅂ',
+        choseong: 'ㄴ',
+        jungseong: 'ㅣ',
+        jongseong: 'ㅂ',
       },
     });
   });
@@ -27,14 +27,14 @@ describe('transformNLAssimilation', () => {
 
     expect(transformNLAssimilation(current, next)).toEqual({
       current: {
-        first: 'ㅇ',
-        middle: 'ㅏ',
-        last: 'ㄹ',
+        choseong: 'ㅇ',
+        jungseong: 'ㅏ',
+        jongseong: 'ㄹ',
       },
       next: {
-        first: 'ㄹ',
-        middle: 'ㅑ',
-        last: 'ㄱ',
+        choseong: 'ㄹ',
+        jungseong: 'ㅑ',
+        jongseong: 'ㄱ',
       },
     });
   });
@@ -45,14 +45,14 @@ describe('transformNLAssimilation', () => {
 
     expect(transformNLAssimilation(current, next)).toEqual({
       current: {
-        first: 'ㅇ',
-        middle: 'ㅑ',
-        last: 'ㅇ',
+        choseong: 'ㅇ',
+        jungseong: 'ㅑ',
+        jongseong: 'ㅇ',
       },
       next: {
-        first: 'ㅇ',
-        middle: 'ㅣ',
-        last: '',
+        choseong: 'ㅇ',
+        jungseong: 'ㅣ',
+        jongseong: '',
       },
     });
   });
diff --git a/src/standardizePronunciation/rules/transformNLAssimilation.ts b/src/standardizePronunciation/rules/transformNLAssimilation.ts
index 3f5d0b93..22910c98 100644
--- a/src/standardizePronunciation/rules/transformNLAssimilation.ts
+++ b/src/standardizePronunciation/rules/transformNLAssimilation.ts
@@ -20,7 +20,7 @@ export function transformNLAssimilation(currentSyllable: Syllable, nextSyllable:
   let next = { ...nextSyllable };
 
   const ㄴㄹ이덧나는조건 =
-    current.last && next.first === 'ㅇ' && arrayIncludes(ㄴㄹ이_덧나는_후속음절_모음, next.middle);
+    current.jongseong && next.choseong === 'ㅇ' && arrayIncludes(ㄴㄹ이_덧나는_후속음절_모음, next.jungseong);
 
   if (!ㄴㄹ이덧나는조건) {
     return {
@@ -41,18 +41,18 @@ function applyㄴㄹ덧남(current: Syllable, next: Syllable): ReturnSyllables {
   const updatedCurrent = { ...current };
   const updatedNext = { ...next };
 
-  if (arrayIncludes(ㄴㄹ이_덧나는_모음, updatedCurrent.middle)) {
-    if (arrayIncludes(ㄴㄹ이_덧나서_받침_ㄴ_변환, updatedCurrent.last)) {
-      updatedCurrent.last = updatedCurrent.last === 'ㄱ' ? 'ㅇ' : updatedCurrent.last;
-      updatedNext.first = 'ㄴ';
+  if (arrayIncludes(ㄴㄹ이_덧나는_모음, updatedCurrent.jungseong)) {
+    if (arrayIncludes(ㄴㄹ이_덧나서_받침_ㄴ_변환, updatedCurrent.jongseong)) {
+      updatedCurrent.jongseong = updatedCurrent.jongseong === 'ㄱ' ? 'ㅇ' : updatedCurrent.jongseong;
+      updatedNext.choseong = 'ㄴ';
     }
 
-    if (arrayIncludes(ㄴㄹ이_덧나서_받침_ㄹ_변환, updatedCurrent.last)) {
-      updatedNext.first = 'ㄹ';
+    if (arrayIncludes(ㄴㄹ이_덧나서_받침_ㄹ_변환, updatedCurrent.jongseong)) {
+      updatedNext.choseong = 'ㄹ';
     }
   } else {
     // ㄴ/ㄹ이 되기 위한 조건이지만 현재 음절의 중성의 ∙(아래아)가 하나가 아닐 경우에는 덧나지 않고 연음규칙이 적용된다
-    updatedNext.first = updatedCurrent.last as typeof updatedNext.first;
+    updatedNext.choseong = updatedCurrent.jongseong as typeof updatedNext.choseong;
   }
 
   return { current: updatedCurrent, next: updatedNext };