diff --git a/book/source/06-signatures.md b/book/source/06-signatures.md
index 466fe09..fb8dc51 100644
--- a/book/source/06-signatures.md
+++ b/book/source/06-signatures.md
@@ -92,7 +92,15 @@ RFC Sections [5.2.3.11](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypt
 
 ## Advanced topics
 
-### Notations
+### Notation signature subpackets
+
+[Notations](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#notation-data) are a signature subpacket type that can be used to effectively extend the otherwise limited set of signature subpacket types with user-defined notations. An issuer can use notations to add name-value data to an OpenPGP signature.
+
+Notation names are UTF-8 encoded strings. They may reside in the "user namespace," which means a notation *tag* (in UTF-8 string format) followed by a DNS domain name.
+
+#### Use of notations by Keyoxide
+
+Notations have, for example, been used for the popular decentralized identity verification service [Keyoxide](https://keyoxide.org/). Keyoxide uses notations in the `ariadne.id` namespace. See the [Keyoxide documentation](https://docs.keyoxide.org/wiki/ariadne-identity/) for more details.
 
 ### "Negotiating" signature hash algorithm based on recipients preference subpackets