diff --git a/CLAUDE.md b/CLAUDE.md
index 88fa614..8bc7e7f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -214,6 +214,17 @@ regardless of which atomic layer it is (a context organism doesn't get its own
 - **Naming:** shared/reusable UI is **English** (language-agnostic: `button`,
   `wizard-shell`); domain contexts are **Dutch** (`registratie`, `herregistratie`,
   `*.machine.ts`). Pick the language by which side of the seam the code is on.
+- **English prose uses Simplified Technical English (STE).** This covers documentation,
+  code comments, commit messages, ADRs, and the backlog notes. One idea per sentence;
+  20 words or fewer in a procedure, 25 in a description. Active voice, present tense.
+  One word for one meaning — pick a term and repeat it, do not vary it for style. Keep
+  articles ("the test fails"). Three nouns together at most. No idioms and no humour.
+  Six sentences per paragraph at most. Write a procedure as numbered steps, one action
+  per step.
+  **STE governs form, not content.** Split a long sentence; never drop a caveat, a
+  measurement, or a precise term to make it shorter.
+  **STE does not apply to** Dutch identifiers, `$localize` copy, quoted output, or
+  existing documents you are not already editing.
 - **User-facing copy = `$localize`.** Every user-visible string is wrapped in Angular's
   first-party `$localize` (no third-party i18n lib), with a stable custom id
   (`` $localize`:@@context.key:Tekst` ``). Source locale is `nl`; a second locale is a