diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index 31bdede..34c745e 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -97,15 +97,15 @@ In the RFC, the OpenPGP primary key is occasionally referred to as "top-level ke
 
 ### Subkeys
 
-In addition to the primary key, modern OpenPGP certificates usually contain a number of "subkeys" (however, it's not technically necessary for a certificate to contain subkeys).
+In addition to the primary key, modern OpenPGP certificates usually contain several subkeys, although they are not technically required.
 
-Subkeys have the same structure as the primary key, but they are used in a different role. Subkeys are cryptographically linked with the primary key (more on this in {numref}`binding_subkeys`).
+Subkeys have the same structural attributes as the primary key but fulfill a different role. Subkeys are cryptographically linked with the primary key (more on this in {numref}`binding_subkeys`).
 
 ```{figure} diag/Subkeys.png
-:name: Certificate with Subkeys
-:alt: Three component keys. The primary key is shown at the top. It can be used for certification. Below it, linked with arrows, are two more component keys, used as subkeys. They are marked as "for encryption" and "for signing", respectively.
+:name: Certificate with subkeys
+:alt: Three component keys depicted. The primary key is positioned at the top, designated for certification. Below it, linked by arrows, are two more component keys, used as subkeys. They are labeled as "for encryption" and "for signing," respectively.
 
-OpenPGP certificates can contain a number of subkeys
+OpenPGP certificates can contain multiple subkeys.
 ```
 
 ### Key flags: defining which operations a component key can perform