Chuck Yeager exceeded the speed of sound in level flight on 14 October 1947, piloting the Bell X-1 "Glamorous Glennis" at Mach 1.06 over the California desert. This milestone has more than symbolic value: it represents the transition to a completely new aerodynamic regime that required dedicated research aircraft, precise instrumentation, and a different testing methodology. Here the chain of flight reaches a concrete physical frontier, just as it had earlier successively solved lift, control, propulsion, and jet propulsion.