diff --git a/source/hw-spec/index.rst b/source/hw-spec/index.rst
index 9b1daff..11923cc 100755
--- a/source/hw-spec/index.rst
+++ b/source/hw-spec/index.rst
@@ -24,3 +24,7 @@ ONI Hardware Specification
    controller/index
 
 
+.. note:: The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+  NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+  "OPTIONAL" in this document are to be interpreted as described in
+  `RFC 2119 <https://datatracker.ietf.org/doc/html/rfc2119>`_. 
\ No newline at end of file