There are two sweep angles of importance, one at the leading edge for supersonic aircraft and the other 25% of the way back from the leading edge for subsonic and transonic aircraft. Leading edge sweep is important because the leading edge has to be behind the mach cone to reduce wave drag.[4] The quarter chord (25%) line is used because subsonic lift due to angle of attack acts there and, up until the introduction of supercritical sections, the crest was usually close to the quarter chord.[5]