diff --git a/book/source/02-highlevel.md b/book/source/02-highlevel.md
index 2e9fab5..5d04713 100644
--- a/book/source/02-highlevel.md
+++ b/book/source/02-highlevel.md
@@ -69,6 +69,12 @@ Today, multiple implementations of OpenPGP play important roles:
 - The Mozilla Thunderbird email software uses [RNP](https://www.rnpgp.org/), its C++ implementation of OpenPGP.
 - The RPM Package Manager software includes an OpenPGP backend based on [Sequoia PGP](https://sequoia-pgp.org/), a modern OpenPGP implementation written in Rust. The Fedora operating system, Fedora Linux 38, [uses Sequoia PGP in rpm](https://sequoia-pgp.org/blog/2023/04/27/rpm-sequoia/).
 
+### Interoperability
+
+OpenPGP was standardized in 1997 to encourage development of interoperable implementations. This has already been a success early on, but in recent years, there has been [much development of new implementations](major_implementations).
+
+Historically, interoperability has only been tested in an adhoc manner. Since 2019, the Sequoia project is maintaining and operating the ["OpenPGP interoperability test suite"](https://tests.sequoia-pgp.org/), for more rigorous and systematic testing. The test suite has identified numerous [issues](https://gitlab.com/sequoia-pgp/openpgp-interoperability-test-suite#hall-of-fame).
+
 ## The road ahead
 ```{admonition} TODO
 Let's insert a line about v5.
@@ -94,7 +100,7 @@ There is [ongoing work](https://datatracker.ietf.org/doc/draft-wussler-openpgp-p
 
 ## Concepts
 ```{admonition} TODO
-Move operations & building blocks statement to Ch1, create Ch2a with the rest 
+Move operations & building blocks statement to Ch1, move certificate and keys section to Ch4 
 ```
 ### Certificates and keys in OpenPGP
 
@@ -121,11 +127,6 @@ With OpenPGP it's possible to:
 To perform these high-level operations, a set of [established cryptographic mechanisms](cyrptography_chapter) are used as building blocks, and combined into OpenPGP's format, which additionally deals with identities and their verification.
 
 (interop_section)=
-## Interoperability
-
-OpenPGP was standardized in 1997 to encourage development of interoperable implementations. This has already been a success early on, but in recent years, there has been [much development of new implementations](major_implementations).
-
-Historically, interoperability has only been tested in an adhoc manner. Since 2019, the Sequoia project is maintaining and operating the ["OpenPGP interoperability test suite"](https://tests.sequoia-pgp.org/), for more rigorous and systematic testing. The test suite has identified numerous [issues](https://gitlab.com/sequoia-pgp/openpgp-interoperability-test-suite#hall-of-fame).
 
 ## Zooming in: Internal structure of OpenPGP data