diff --git a/index.js b/index.js
index 182e447..743c11a 100644
--- a/index.js
+++ b/index.js
@@ -5,3 +5,4 @@ module.exports.Validator = require('./validator.js');
 module.exports.Parser = require('./parser.js');
 module.exports.Tracker = require('./tracker.js');
 module.exports.Reader = require('./reader.js');
+module.exports.Letterbox = require('./reader.js');
diff --git a/letterbox.js b/letterbox.js
new file mode 100644
index 0000000..05afc93
--- /dev/null
+++ b/letterbox.js
@@ -0,0 +1,114 @@
+/**
+ * @author Tom De Caluwé
+ * @copyright 2016 Tom De Caluwé
+ * @license Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict'
+
+/**
+ * Construct a new letterbox accepting EDIFACT envelopes. A letterbox is a
+ * Writable stream accepting segment objects. Enveloping is optional, so a
+ * single message will also be accepted. Groups inside an envelope are optional
+ * as well. When used however, every message should be in a group.
+ *
+ * @constructs Letterbox
+ */
+var Letterbox = function () {
+  var letterbox = this;
+
+  this.depth = {};
+  this.depth.current = 0;
+  this.depth.minimum = 0;
+  this.depth.maximum = 2;
+
+  this.next = function () {
+    letterbox.depth.current -= 1;
+  };
+}
+
+module.exports = Letterbox;
+
+function open_envelope(name, level) {
+  var message = '';
+  var depth = this.depth.current + 1;
+
+  if (this.depth.current !== level) {
+    message += 'Cannot open the ' + name + ' at the current enveloping level';
+    throw Error(message);
+  } else if (depth > this.depth.maximum) {
+    message += 'Cannot open the ' + name + ' since it has been omitted before';
+    throw Error(message);
+  } else {
+    this.depth.current = depth;
+    this.depth.minimum = depth;
+  }
+}
+
+function close_envelope(name, level) {
+  var message = '';
+  var depth = this.depth.current - 1;
+
+  if (depth !== level) {
+    message += 'Cannot close the ' + name + ' at the current enveloping level';
+    throw Error(message);
+  } else {
+    this.depth.current = depth;
+  }
+}
+
+/**
+ * Accept a segment.
+ *
+ * @param {String} segment A segment object.
+ */
+Letterbox.prototype.write = function (segment) {
+  // Most of the time we are tracking segments in a message. To optimize for
+  // this case we start by detecting if we are currently in the middle of a
+  // message. We can do this with only one comparison.
+  if (this.depth.current > this.depth.maximum) {
+    this.track(segment);
+  } else {
+    switch (segment.name) {
+    case 'UNB':
+      open_envelope.call(this, 'interchange', 0);
+      break;
+    case 'UNG':
+      open_envelope.call(this, 'group', 1);
+      break;
+    case 'UNH':
+      if (this.depth.current < this.depth.minimum) {
+        throw Error('Cannot omit an envelope');
+      } else {
+        this.depth.maximum = this.depth.current;
+        this.depth.current += 1;
+        this.track(segment);
+      }
+      break;
+    case 'UNE':
+      close_envelope.call(this, 'group', 1);
+      break;
+    case 'UNZ':
+      close_envelope.call(this, 'interchange', 0);
+      break;
+    default:
+      throw Error('Did not expect a ' + segment + ' segment');
+    }
+  }
+};
+
+Letterbox.prototype.track = function () {};
+
+module.exports = Letterbox;
diff --git a/spec/LetterboxSpec.js b/spec/LetterboxSpec.js
new file mode 100644
index 0000000..7096f99
--- /dev/null
+++ b/spec/LetterboxSpec.js
@@ -0,0 +1,85 @@
+'use strict'
+
+let Letterbox = require('../letterbox.js');
+
+describe('Letterbox', function () {
+  let letterbox;
+  beforeEach(function () {
+    letterbox = new Letterbox();
+    spyOn(letterbox, 'track');
+  });
+  it('should accept a message without an envelope', function () {
+    expect(function () { letterbox.write({ name: 'UNH' }); }).not.toThrow();
+    expect(letterbox.depth.maximum).toEqual(0);
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.next(); }).not.toThrow();
+    expect(letterbox.depth.current).toEqual(0);
+  });
+  it('cannot nest interchanges', function () {
+    expect(function () { letterbox.write({ name: 'UNB' }); }).not.toThrow();
+    expect(letterbox.depth.minimum).toEqual(1);
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNB' }); }).toThrow();
+  });
+  it('cannot open an interchange after a single message', function () {
+    expect(function () { letterbox.write({ name: 'UNH' }); }).not.toThrow();
+    expect(letterbox.depth.maximum).toEqual(0);
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.next(); }).not.toThrow();
+    expect(letterbox.depth.current).toEqual(0);
+    expect(function () { letterbox.write({ name: 'UNB' }); }).toThrow();
+  });
+  it('cannot open a group without an interchange', function () {
+    expect(function () { letterbox.write({ name: 'UNG' }); }).toThrow();
+  });
+  for (var name of ['UNB', 'UNZ', 'UNG', 'UNE']) {
+    it('should not validate a ' + name + ' segment while tracking a message', function () {
+      // While no valid message contains an enveloping segment in it's segment
+      // table, this test is the equivalence of allowing such a messsage
+      // definition. Prohibiting enveloping segments in messages should be
+      // done by providing a correct segment table, not by algorithm design.
+      expect(function () { letterbox.write({ name: 'UNH' }); }).not.toThrow();
+      expect(letterbox.depth.maximum).toEqual(0);
+      expect(letterbox.depth.current).toEqual(1);
+      expect(function () { letterbox.write({ name: name }); }).not.toThrow();
+      expect(letterbox.track).toHaveBeenCalled();
+    });
+  }
+  it('should accept a message in an interchange', function () {
+    expect(function () { letterbox.write({ name: 'UNB' }); }).not.toThrow();
+    expect(letterbox.depth.minimum).toEqual(1);
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNH' }); }).not.toThrow();
+    expect(letterbox.depth.maximum).toEqual(1);
+    expect(letterbox.depth.current).toEqual(2);
+    expect(function () { letterbox.next(); }).not.toThrow();
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNZ' }); }).not.toThrow();
+    expect(letterbox.depth.current).toEqual(0);
+  });
+  it('should not accept a group after a message', function () {
+    expect(function () { letterbox.write({ name: 'UNB' }); }).not.toThrow();
+    expect(letterbox.depth.minimum).toEqual(1);
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNH' }); }).not.toThrow();
+    expect(letterbox.depth.maximum).toEqual(1);
+    expect(letterbox.depth.current).toEqual(2);
+    expect(function () { letterbox.next(); }).not.toThrow();
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNG' }); }).toThrow();
+  });
+  it('should not accept a group without an interchange', function () {
+    expect(function () { letterbox.write({ name: 'UNG' }); }).toThrow();
+  });
+  it('should not accept a message after a group', function () {
+    expect(function () { letterbox.write({ name: 'UNB' }); }).not.toThrow();
+    expect(letterbox.depth.minimum).toEqual(1);
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNG' }); }).not.toThrow();
+    expect(letterbox.depth.minimum).toEqual(2);
+    expect(letterbox.depth.current).toEqual(2);
+    expect(function () { letterbox.write({ name: 'UNE' }); }).not.toThrow();
+    expect(letterbox.depth.current).toEqual(1);
+    expect(function () { letterbox.write({ name: 'UNH' }); }).toThrow();
+  });
+});