diff --git a/docs/v4.0.0/ENGINE.md b/docs/v4.0.0/ENGINE.md
index c6b6471c..060509a9 100644
--- a/docs/v4.0.0/ENGINE.md
+++ b/docs/v4.0.0/ENGINE.md
@@ -295,6 +295,15 @@ Rulings on that step so far:
   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.
+- **The geometry is data, not design (confirmed, "yes").** A node has a
+  number of ports, and that number is a parameter, as cell width and node
+  memory are. Which port connects to what is a table that can change while
+  the system runs. A geometry is a rule for filling in that table; the
+  units-and-centres rule below is the first, not the only one. Devices —
+  the common SSD, the console, a node's own drive — are things on the
+  other end of a port, in the same table. Nothing in the engine knows
+  which geometry is in use. A node is told, or finds out, which port leads
+  toward a destination.
 - **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