diff --git a/core/lib/Drupal/Core/Cache/CacheBackendInterface.php b/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
index 753e6a3..2960db5 100644
--- a/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
+++ b/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
@@ -14,6 +14,9 @@
  * Drupal\Core\Cache\DatabaseBackend provides the default implementation, which
  * can be consulted as an example.
  *
+ * The cache indentifiers are case sensitive. That is access to caches with
+ * cid's 'APPLE' and 'apple' are seperate and distinct.
+ *
  * @ingroup cache
  */
 interface CacheBackendInterface {