diff --git a/user/CLAUDE.md b/user/CLAUDE.md
index 90db087..5a34e1c 100644
--- a/user/CLAUDE.md
+++ b/user/CLAUDE.md
@@ -17,6 +17,26 @@ Known substitutions seen so far:
 
 Add new ones to this table as they come up.
 
+## Writing style
+
+Write in **Simplified Technical English**, and follow **Zinsser's four
+principles**: clarity, simplicity, brevity, humanity.
+
+In practice:
+
+- **Short sentences.** One idea each.
+- **Active voice.** Name who does the thing.
+- **One meaning per word.** Pick a term, then reuse it. Do not vary the wording
+  for elegance — in technical writing a synonym reads as a second thing.
+- **Cut the clutter.** Remove any word that carries no weight. Most qualifiers,
+  most adverbs, and every phrase that only restates the sentence before it.
+- **Stay warm and human.** A person wrote this, not a manual. Plain does not
+  mean cold, and brief does not mean blunt.
+
+This governs prose written for the user: explanations, summaries, reports,
+commit messages, and comments. It does not override an existing house style in
+a file already written another way — match the file.
+
 ## Ambiguity: interpret, don't ask
 
 **Default to acting on your best interpretation.** Work out what was meant from