diff --git a/book/source/13-armor.md b/book/source/13-armor.md
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@@ -1,7 +1,31 @@
 # ASCII Armor
 
-```
-  - Why?
-  - CRC and [its deprecation in crypto-refresh](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-optional-checksum)
+The native format of OpenPGP data is binary.
+
+However, in many use cases it is customary to use OpenPGP data in a non-binary encoding called "ASCII Armor." For example, ASCII Armored OpenPGP data is often used in email, for encrypted messages or for signatures.
+
+OpenPGP's ASCII Armor mechanism consists of:
+
+- A [header line](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-armor-header-line)
+- [Headers](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-armor-headers) that can contain additional metadata
+- The [Base64 encoded](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-base64-conversions) OpenPGP data
+  - An optional checksum for this data
+- A ["tail line"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-armor-tail-line) (footer) that matches the header line
+
+## The Cleartext Signature Framework
+
+```{admonition} TODO
+:class: warning
+
+Explain/discuss, [link](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-cleartext-signature-framewo)
+
   - Linebreak normalization?
-```
\ No newline at end of file
+  - Dash escaping
+```
+
+
+## Advanced topics
+
+### CRC (and its deprecation in crypto-refresh)
+
+See https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-optional-checksum