diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index c537fec..f3e221a 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -362,7 +362,7 @@ We have to distinguish the "packet level" information about a certificate from a
 
 #### Reasoning about append-only properties in a distributed system
 
-OpenPGP is a thoroughly distributed system. Users can obtain and transmit certificate information about their own, as well as other users', certificates using a broad range of mechanisms. These mechanisms include keyservers, manual handling, WKD and [Autocrypt](https://en.wikipedia.org/wiki/Autocrypt).
+OpenPGP is a thoroughly distributed system. Users can obtain and transmit certificate information about their own, as well as other users', certificates using a broad range of mechanisms. These mechanisms include keyservers, manual handling, [Web Key Directory](https://datatracker.ietf.org/doc/draft-koch-openpgp-webkey-service/) (WKD) and [Autocrypt](https://en.wikipedia.org/wiki/Autocrypt).
 
 User's OpenPGP software may obtain different views of a particular certificate, over time. These systems have to reconcile and store a combined version of the possibly disparate elements they may obtain from different sources.