diff --git a/index.html b/index.html
index 7278cc8..3a6678c 100644
--- a/index.html
+++ b/index.html
@@ -227,6 +227,11 @@
     <dfn>internal slots</dfn></a> of an object, instead of saying
     <i>"the [[\y]] internal slot of x"</i>.
   </p>
+  <p class="note">
+    Internal slots are used only as a notation in this specification, and
+    implementations do not necessarily have to map them to explicit internal
+    properties.
+  </p>
   <p>
     The Augmented Backus-Naur Form (ABNF) notation used is specified in
     [[RFC5234]].