diff --git a/docs/v4.0.0/ENGINE.md b/docs/v4.0.0/ENGINE.md
index 6106c1b9..c6b6471c 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.
+- **The geometry is not fixed: "Not constrained by a 3D world. Other
+  geometries might be better."** Said when asked what a centre's two
+  remaining ports of six were for. So six, which came from a 12^3 grid's
+  six neighbours, is not a given either; nor is any one shape.
 - **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