diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index e80769b..8d5e787 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -83,6 +83,8 @@ The "OpenPGP primary key" is a component key that serves a central role in an Op
 - Its fingerprint is used as the unique identifier for the full OpenPGP certificate.
 - It is used for lifecycle operations, such as adding or invalidating subkeys or identities in a certificate.
 
+The validity of the primary key limits its capacity to confer validity to other components. E.g.: The primary key cannot confer an expiration time beyond its own expiration to a subkey. It can also not confer validity to components after it has been revoked.
+
 ```{admonition} Terminology
 :class: note