diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 9c274c5..ee2dfca 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -592,11 +592,15 @@ pub struct Session<G> { /* replayable instruction log + derived stats */ }
 
 // From `klondike` (upstream — never edit):
 pub enum KlondikePile {
-    Stock,
-    Waste,
+    Stock,                    // NB: no Waste variant — see below
     Foundation(Foundation),   // 4 slots, any suit may claim any slot
     Tableau(Tableau),         // 7 columns
 }
+// Pile-coordinate convention: upstream has no `Waste` variant. In
+// pile-coordinate space `KlondikePile::Stock` denotes the face-up
+// *waste* pile (the only stock-side pile cards move out of); use
+// `GameState::stock_cards()` / `waste_cards()` when the face-down
+// draw stack must be distinguished. Documented on `GameState::pile`.
 pub enum DrawStockConfig { DrawOne, DrawThree }
 pub enum KlondikeInstruction { /* RotateStock, DstFoundation, ... — the
                                   serialized move format (schema v4+) */ }