From 7d8a6f984a6bb59e82309cc86c4593174bf2898f Mon Sep 17 00:00:00 2001
From: Armin Ruech <7052238+arminru@users.noreply.github.com>
Date: Tue, 19 Dec 2023 19:00:17 +0100
Subject: [PATCH] Clarify that attribute keys are case-sensitive (#3784)

---
 CHANGELOG.md                   | 3 +++
 specification/common/README.md | 1 +
 2 files changed, 4 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index daba7b5387e..a84b2c15fea 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,9 @@ release.
 
 ### Common
 
+- Clarify that attribute keys are case-sensitive.
+  ([#3784](https://github.com/open-telemetry/opentelemetry-specification/pull/3784))
+
 ### Supplementary Guidelines
 
 ## v1.28.0 (2023-12-07)
diff --git a/specification/common/README.md b/specification/common/README.md
index 1b46da3e628..4b9adc77b2d 100644
--- a/specification/common/README.md
+++ b/specification/common/README.md
@@ -28,6 +28,7 @@ aliases: [/docs/reference/specification/common/common]
 An `Attribute` is a key-value pair, which MUST have the following properties:
 
 - The attribute key MUST be a non-`null` and non-empty string.
+  - Case sensitivity of keys is preserved. Keys that differ in casing are treated as distinct keys.
 - The attribute value is either:
   - A primitive type: string, boolean, double precision floating point (IEEE 754-1985) or signed 64 bit integer.
   - An array of primitive type values. The array MUST be homogeneous,