diff --git a/book/source/18-zoom_private_keys.md b/book/source/18-zoom_private_keys.md
index d6a8acb..095850d 100644
--- a/book/source/18-zoom_private_keys.md
+++ b/book/source/18-zoom_private_keys.md
@@ -17,7 +17,16 @@ $ sq packet dump --hex alice.priv
 
 ### Primary Secret-Key packet
 
-The output starts with the (primary) [Secret-Key packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-packet-formats):
+The output starts with the (primary) [Secret-Key packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-packet-formats).
+
+This is the structure of the Secret-Key packet we will now look at.
+
+```{figure} diag/secret-key_packet.png
+
+Structure of a Secret-Key packet.
+```
+
+The output of Sequoia's `sq packet dump` for this packet:
 
 ```text
 Secret-Key Packet, new CTB, 2 header bytes + 75 bytes
diff --git a/book/source/diag/secret-key_packet.png b/book/source/diag/secret-key_packet.png
new file mode 100644
index 0000000..c10c1b6
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