diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index 60da4d6..c024a47 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -644,7 +644,7 @@ In the following examples, we will only look at OpenPGP keys that include the pr
 
 ### Encryption subkey
 
-Now we'll look at a subkey in Alice's key. An OpenPGP subkey, when it is linked to an OpenPGP certificate, effectively consists of two elements:
+Now we'll look at a subkey in Alice's key. An OpenPGP subkey, when it is linked to an OpenPGP certificate (via its primary key), effectively consists of two elements:
 
 - a key packet that contains the component key itself, and
 - a signature packet that links this component key to the primary key (and thus implicitly to the full OpenPGP certificate).