diff --git a/docs/rules/DEV-230.md b/docs/rules/DEV-230.md
index 5bebb30..493f600 100644
--- a/docs/rules/DEV-230.md
+++ b/docs/rules/DEV-230.md
@@ -25,17 +25,23 @@ applied to every issue, PR description, comment, review reply and chat message.
    own hesitation or worry.
 1. Cut openers that announce a point instead of making it, such as "Worth
    noting", "Just to add" or "Good question".
-1. End with what you need, from whom. When nothing is needed, stop.
+1. End with what you need, from whom. When nothing is needed, stop. A hedged
+   closer such as "Feel free to ask" or "Let me know if you have any questions"
+   asks for nothing, so cut it.
+1. Join clauses with a comma, colon, period or parentheses, which say how the
+   parts relate. An em dash does not show how the parts relate. Use an en dash only in a number range.
 
 ```md
 Good: Blocked on the MAS field list. @owner, which company fields does MAS
       receive? I need it to finish the edit gate.
 Bad:  Hi @owner, hope you're well! Great work on the spec so far. I've been
       looking into the edit gate and I'm a bit worried about the fields...
+      Let me know if you have any questions.
 ```
 
 ### Acceptance Criteria
 
 - [ ] The first line carries the decision, answer, status or question
-- [ ] The message holds no greeting, flattery or personal aside
+- [ ] The message holds no greeting, flattery, personal aside or hedged closer
 - [ ] A message that needs something from someone names what and from whom
+- [ ] No em dash appears in prose, and an en dash only inside a number range
diff --git a/docs/rules/DEV-260.md b/docs/rules/DEV-260.md
new file mode 100644
index 0000000..80c7533
--- /dev/null
+++ b/docs/rules/DEV-260.md
@@ -0,0 +1,41 @@
+---
+id: DEV-260
+title: "Define Every Term the Reader Has Not Seen"
+status: "active"
+enforcement: "manual"
+severity: "warning"
+depends_on: ["DEV-230"]
+---
+
+## Problem
+
+A message built on words the reader has not seen defined, such as "middle
+ground" or "eng-month", makes them guess, and two readers guess differently.
+
+## Solution
+
+A word earns its place only if it survives the question "what does that mean
+here?". The writer knows what they meant; the reader only has the page.
+
+1. Replace an abstract verb such as "align", "streamline", "formalise" or
+   "leverage" with the action it stands for.
+1. Replace a catch-all such as "everything else" or "the rest" with the list it
+   covers.
+1. Define a term the first time you use it, unless the thread or a linked doc
+   already defines it. Otherwise replace it with the concrete thing.
+1. Refer to an earlier position by stating it or linking it, not by a shorthand
+   only you hold.
+
+```md
+Good: A month with under 40 logged hours bills by the days worked, not as a
+      full month.
+Bad:  This keeps partner protection on light months (no "16h = full month").
+```
+
+### Acceptance Criteria
+
+- [ ] Every unfamiliar term is defined in the message, the thread, or a linked
+  doc
+- [ ] No abstract verb stands in for an action that can be named
+- [ ] No catch-all stands in for a list that can be given
+- [ ] Every earlier position referred to is stated or linked
diff --git a/docs/rules/README.md b/docs/rules/README.md
index 736b060..050c008 100644
--- a/docs/rules/README.md
+++ b/docs/rules/README.md
@@ -67,6 +67,7 @@ Where discussion goes, how a message is written, and how work is referenced.
 - [DEV-240](./DEV-240.md): mention someone only to ask them to act
 - [DEV-245](./DEV-245.md): post a daily update on the work you own
 - [DEV-250](./DEV-250.md): settle it async before calling a meeting
+- [DEV-260](./DEV-260.md): define every term the reader has not seen
 
 ### 3. PR requirements