diff --git a/docs/v4.0.0/ENGINE.md b/docs/v4.0.0/ENGINE.md
index 3ab890d1..6106c1b9 100644
--- a/docs/v4.0.0/ENGINE.md
+++ b/docs/v4.0.0/ENGINE.md
@@ -291,6 +291,10 @@ Rulings on that step so far:
   what a 12^3 grid needs. `DECOMPOSITION.md` section 6 is to be corrected.
 - **It must scale at run time, adaptively.** The number of nodes and their
   wiring are not fixed when the system is built.
+- **How it grows: "Only the central node can connect to only another
+  central node."** So the five are a unit: four outer nodes and their
+  centre. An outer node is wired only inside its own unit. Units are joined
+  centre to centre, and the system grows by units.
 
 The design of this step is being worked out by question and answer and is
 not written yet. Nothing of it is built.