A TCAS maneuver was involved. TCAS is the Traffic Collision and Avoidance System. It uses transponder returns from airplanes equipped with Mode C transponders to calculate proximity and closure rate. When a pilot receives a Resolution Advisory, the system provides explicit escape guidance that the pilot is obligated to follow. The system does not work when the other airplane does not have a transponder or when the transponder is turned off. The airspace system is structured to attempt to keep such airplanes out of the airspace normally used by airliners.