On a typical aerofoil, a point exists about which the resultant aerodynamic pitching moment M is independent of incidence. This special point is called the Aerodynamic Centre (AC). The numerical value of the pitching moment about AC is denoted M_AC. For standard aerofoils in subsonic flows, the Aerodynamic Centre lies approximate ¼ of the chord behind the leading edge (i.e. h=x_AC/c ≈ ¼). For supersonic flows, the AC moves abruptly aft to around h ≈ ½; see Fig. 2.2-1. Since M_AC is approximately constant over a wide range of C_L including zero, MAC is sometimes referred to as the zero-lift pitching moment and denoted M₀.