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"Mach 1": The term refers to the speed of sound, which is not a fixed number but varies based on the temperature of the surrounding gas. Named for Austrian physicist Ernst Mach, the term is a little science-geeky, but Ford evidently felt it connoted jet-age performance. Ford first applied the Mach 1 name to a Mustang with a 1968 show car that featured a chopped roofline and square headlights. Neither of those elements made it into the production version that arrived with the aggressively restyled new Mustang for 1969.