Definition
Inventory held to decouple Monument from one-piece flow; quantity = external process delivery time ÷ takt, rounded up. Two official notes ( p84): ① delivery time must include the round-trip transport time between the cell and the external process; ② when the Monument equipment is a batch process, add ×2 to the Curtain quantity. "Double when FIFO is not feasible" is another third-party (Rexnord) definition and must not be mixed with these two notes.
In detail
The Curtain is inventory held to decouple the Monument from one-piece flow, with quantity equal to the external process lead time divided by takt time and rounded up. It solves the pace mismatch between large equipment and cellular production, ensuring cells do not stall while waiting. The Curtain is a buffer stock; unlike Safety Stock, it is designed specifically for equipment bottlenecks. The output is an explicit inventory quantity and replenishment rule. The glossary also has a separate "Curtain Effect" entry describing the same thing, and the two once gave different numerators (this entry uses external process lead time; that one uses the Curtain operation's one-piece cycle time). Before citing, verify which version is being used; see definition for the two official REV3 notes (transport time counts toward lead time; batch Monument doubling).
Example in practice
A cell needs to take parts from a shared heat-treat furnace. The furnace outputs a batch every two hours, the round-trip transport takes 10 minutes, and the cell takt time is 10 minutes. Delivery time = 120 + 10 = 130 minutes, 130/10 rounded up = 13 pieces; the heat-treat furnace is a batch-process Monument, plus ×2 = 26 pieces. In this way, the cell can produce continuously while the furnace runs in batches, and the two do not interfere with each other.