diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
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--- a/book/source/04-certificates.md
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@@ -1,7 +1,7 @@
 (certificates_chapter)=
 # Certificates
 
-One central (and non-trivial) element of OpenPGP are "OpenPGP certificates" (also often called "OpenPGP keys"). OpenPGP certificates are relatively complex data structures, so it's good to have a clear mental model of them.
+OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," often referred to as "OpenPGP keys." These certificates are complex data structures essential for identity verification and data encryption. Understanding their structure and functionality is pivotal for effective application of the OpenPGP standard.
 
 ## Terminology: The various meanings of "key"