diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index b0119ad..b99fe94 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -176,7 +176,7 @@ For example, the capabilities of a component key, such as *signing* or *encrypti
 Instead, this kind of metadata is stored as part of the signature packets that join components into an OpenPGP certificate:
 
 - For the primary key, its key flags and other metadata can be defined in two ways:
-  - With a [direct key signature](direct_key_signature) on the primary key,
+  - With a [direct key signature](direct_key_signature) on the primary key (preferred method in OpenPGP version 6),
   - or by associating the metadata with the [Primary User ID](primary_user_id).
 - For subkeys, metadata is defined with the [subkey binding signature](binding_subkeys) that links the subkey to the certificate.
 - For identity components like User IDs, metadata is associated via the [certifying self-signature](bind_ident) that links the identity to the certificate.