diff --git a/book/source/02-highlevel.md b/book/source/02-highlevel.md
index 8c50f89..ef83898 100644
--- a/book/source/02-highlevel.md
+++ b/book/source/02-highlevel.md
@@ -55,7 +55,7 @@ Because the GnuPG program binary is called "gpg," "GnuPG" and "gpg" are often us
 
 In 2007, the IETF published [RFC 4880](https://datatracker.ietf.org/doc/html/rfc4880), which defines version 4 OpenPGP artifacts. As of late 2023, version 4 is the most commonly used version.
 
-An extension for Elliptic Curve Cryptography was defined in [RFC 6637](https://www.rfc-editor.org/rfc/rfc6637).
+An extension for Elliptic Curve Cryptography was defined in [RFC 6637](https://www.rfc-editor.org/rfc/rfc6637), specifying the use of three NIST prime field curves.
 
 Some implementations explored other non-standardized extensions. Notably, algorithms based on Curve 25519 were tentatively defined in the [rfc4880bis](https://www.ietf.org/archive/id/draft-ietf-openpgp-rfc4880bis-10.html#name-elliptic-curve-cryptography) document. These algorithms are widely used, even though rfc4880bis has never been finalized as a new version of the standard.