diff --git a/book/source/02-highlevel.md b/book/source/02-highlevel.md
index e455683..832bc3d 100644
--- a/book/source/02-highlevel.md
+++ b/book/source/02-highlevel.md
@@ -37,7 +37,7 @@ The original PGP software was never under a Free Software license, despite its s
 
 While PGP was first developed as commercial software, the owner at the time, PGP Inc., started a standardization effort with the IETF, first publishing [RFC 1991 "PGP Message Exchange Formats"](https://datatracker.ietf.org/doc/html/rfc1991) in August 1996.
 
-In July 1997, a process to produce an open standard under the then new name [OpenPGP](https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP) was started, resulting in [RFC 2440 "OpenPGP Message Format"](https://datatracker.ietf.org/doc/html/rfc2440), published in November 1998. RFC 2440 describes OpenPGP version 3.
+In July 1997, a process to produce an open standard under the then new name [OpenPGP](https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP) was started, resulting in [RFC 2440 "OpenPGP Message Format"](https://datatracker.ietf.org/doc/html/rfc2440), published in November 1998.
 
 The name OpenPGP can be used freely by implementations, unlike the name PGP, which is a [registered trademark](https://uspto.report/TM/74685229).
 
@@ -49,11 +49,15 @@ GnuPG was a major early implementation of OpenPGP. Over the years, the importanc
 
 Because the GnuPG program binary is called "gpg," "GnuPG" and "gpg" are often used interchangeably.
 
-## The OpenPGP version 4 era
+## The RFC 4880 era
 
 ### OpenPGP version 4
 
-In 2007, the IETF published [RFC 4880](https://datatracker.ietf.org/doc/html/rfc4880), which defined version 4 of the OpenPGP standard. As of late 2023, version 4 is the most commonly used version.
+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).
+
+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.
 
 (major_implementations)=
 ### Major implementations of OpenPGP