American Standard M952100-0070A Pressure Balancing Unit for Single-Control Tub/Shower Valve;

This pressure balancing unit maintains a constant output in response to changes in the hot and cold water supply pressure. This reduces the chance you'll experience dramatic changes in water temperature in the shower. The diaphragm system with integrated check valves is engineered to eliminate cross flow and this Pressure Balance Unit is also designed to avoid failure due to mineral deposits, so it is ideal for use in hard water areas. This pressure balance protects against scalding and is for use with American Standard R110, R120, R115, R125 rough in valves.