diff --git a/03-DESIGN/01-to-be/42-the-machines-modules-in-order.md b/03-DESIGN/01-to-be/42-the-machines-modules-in-order.md
index 1e8d37b..82b850e 100644
--- a/03-DESIGN/01-to-be/42-the-machines-modules-in-order.md
+++ b/03-DESIGN/01-to-be/42-the-machines-modules-in-order.md
@@ -15,6 +15,7 @@ decisions:
   - 02-DECISIONS/0205-software-the-distribution-does-not-package-ships-as-a-pinned-archive-of-the-module.md
   - 02-DECISIONS/0207-a-module-depends-on-the-node-seats-that-apply-its-resources.md
   - 02-DECISIONS/0208-the-graphical-session-is-one-module-per-piece-on-the-meshs-seats.md
+  - 02-DECISIONS/0210-a-tools-configuration-is-its-seat-holders-and-every-other-module-extends-it-through-the-seat.md
 ---
 
 # 42. The machines' modules, in order
@@ -93,6 +94,13 @@ In order:
 
 The seats, gating, contributions and session start are [ADR 0208](../../02-DECISIONS/0208-the-graphical-session-is-one-module-per-piece-on-the-meshs-seats.md).
 
+**Who writes what** is [ADR 0210](../../02-DECISIONS/0210-a-tools-configuration-is-its-seat-holders-and-every-other-module-extends-it-through-the-seat.md): a tool's configuration belongs to the
+holder of its seat. The launcher, the clipboard manager, the wallpaper and the bar contribute their
+window-manager lines to `node-display-session`, and the window manager's module places them. They do
+not write into its include directory. Each contribution is a dependency on the seat that receives it,
+so assigning one of them without a window manager is refused. The first versions, which still write
+the include files themselves, move to contributions once the controller derives the dependency.
+
 ## Phase 3 — one machine model
 
 The laptop's hardware module (vendor daemon, GPU mode, charge limit, logind, brightness and vendor keys)