Q-Corner or Coffin Corner is a term used to describe operations at high altitudes where low indicated airspeeds yield high true airspeeds (MACH number) at high angles of attack. The high angle of attack results in flow separation which causes buffet. Turning maneuvers at these altitudes increase the angle of attack and result in stability deterioration with a decrease in control effectiveness. The relationship of stall speed to MACH crit narrows to a point where sudden increases in angle of attack, roll rates, and/or disturbances (e.g., clear air turbulence) cause the limits of the airspeed envelope to be exceeded. Coffin Corner exists in the upper portion of the maneuvering envelope for a given gross weight and G-force.