diff --git a/book/source/09-verification.md b/book/source/09-verification.md
index ae400f2..8691c74 100644
--- a/book/source/09-verification.md
+++ b/book/source/09-verification.md
@@ -20,6 +20,7 @@ A signature might be correct, but still disqualify as a valid signature.
 Put mathematically, the set of valid signatures is a subset of the set of correct signatures.
 
 The validity of a correct signature is additionally constrained by a number of conditions:
+Signatures need to be well-formed, meaning they must contain required signature subpackets in the proper subpacket area and must not contain unknown critical subpackets or notations.
 Most signatures have a limited validity period, constrained by the signature creation- and expiration time.
 Furthermore, some signatures need to be *qualified* by another valid signature in order to be considered valid.
 Lastly, signatures can be invalidated by revocations.