Refer to figure.
Stall boundaries set a lower limit to the operating speed, according to the load factor. In the case of a high-speed aircraft, there is also an upper limit which is due to the approach of shock stall and the associated buffet which occurs if the aircraft enters the transonic speed range. The limits associated with these effects give the buffet boundaries.

The margin between the two types of stall narrows with increasing altitude. Notably the true airspeed of the low-speed stall increases with increasing altitude for a given indicated airspeed, whilst the true airspeed of the shock stall reduces up to the base of the tropopause, above which it remains constant.

The point at which the two stalls coincide is referred to as the 'coffin corner'. The altitude at which an aircraft can fly at one airspeed in a 1 g maneuver is called the aerodynamic ceiling. Since this condition has no safety margin, aircraft must be operated within a buffet margin of 0.3 g.