diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index 34c745e..795a741 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -110,6 +110,10 @@ OpenPGP certificates can contain multiple subkeys.
 
 ### Key flags: defining which operations a component key can perform
 
+```{admonition} Warning
+Let's decide whether the capitalization of F is necessary.
+```
+
 Each component key has a set of ["Key Flags"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-flags) that specify which operations that key can perform.
 
 The commonly used key flags are: