diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index 18f20b9..1e8110f 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -326,9 +326,11 @@ Component keys use *Key Expiration Time* subpackets for expressing the expiratio
 
 Since OpenPGP certificates act as ["append only" data structures](append-only), existing components or signatures cannot simply be "removed." Instead, they can be marked as invalid by issuing revocation signatures. These additional revocation signatures are added to the certificate.
 
-Each component, such as User ID and a subkey, may be revoked without affecting the rest of the certificate.
+Each component, such as User ID and a subkey, can be revoked without affecting the rest of the certificate.
 
-Revoking the primary key with a [*Key revocation signature*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-key-revocation-signature-ty) (type ID `0x20`) is a special case: This marks the entire certificate, including all of its components unusable.
+The *primary User ID* is an exception: when it is revoked, the entire certificate is considered invalid.
+
+Revoking the primary key with a [*Key revocation signature*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-key-revocation-signature-ty) (type ID `0x20`) also marks the entire certificate, including all of its components, as invalid and unusable.
 
 #### Semantics of Revocations