diff --git a/README.md b/README.md
index 72c8fccd3..419eeb5d9 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,8 @@ The combination of the image manifest, image configuration, and one or more file
 
 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC 2119](http://tools.ietf.org/html/rfc2119) (Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997).
 
+The keywords "unspecified", "undefined", and "implementation-defined" are to be interpreted as described in the [rationale for the C99 standard][unspecified].
+
 ![](img/build-diagram.png)
 
 Once built the OCI Image can then be discovered by name, downloaded, verified by hash, trusted through a signature, and unpacked into an [OCI Runtime Bundle](https://github.com/opencontainers/runtime-spec/blob/master/bundle.md).
@@ -176,3 +178,4 @@ Read more on [How to Write a Git Commit Message](http://chris.beams.io/posts/git
 
 [UberConference]: https://www.uberconference.com/opencontainers
 [irc-logs]: http://ircbot.wl.linuxfoundation.org/eavesdrop/%23opencontainers/
+[unspecified]: http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf#page=18
diff --git a/layer.md b/layer.md
index b441b8dba..6e771cc84 100644
--- a/layer.md
+++ b/layer.md
@@ -277,6 +277,6 @@ Typically, such layers are downloaded directly from a distributor but are never
 
 Layers that have these restrictions SHOULD be tagged with an alternative mediatype of `application/vnd.oci.image.layer.nondistributable.tar+gzip`.
 [Descriptors](descriptor.md) referencing these layers MAY include `urls` for downloading these layers.
-It is implementation defined whether or not implementations upload layers tagged with this media type.
+It is implementation-defined whether or not implementations upload layers tagged with this media type.
 
 [tar-archive]: https://en.wikipedia.org/wiki/Tar_(computing)