Wind instruments exhibit overblowing: http://en.wikipedia.org/wiki/Overblowing Naively, I would expect that if one, for example, blew harder on a whistle, or blew at a different angle on a flute, the result would be as follows. You produce some noise spectrum, which becomes a different noise spectrum when you change how you blow. This noise spectrum drives the air column, which has a series of discrete resonances at frequencies fo, 2fo, 3fo, ... (or possibly only the odd multiples, if the boundary conditions are asymmetric). The air column responds strongly at those frequencies, with amplitudes that are proportional to the noise spectrum at those frequencies and also proportional to the strengths of the resonances. The spectrum that results contains frequencies fo, 2fo, 3fo, ..., and overblowing only changes the relative strengths of the harmonics, which means there is only a change in timbre, not a change in pitch. The pitch still corresponds to that of the fundamental fo. The timbre varies continuously as a function of how you blow. It should be impossible to produce a change in pitch without the use of a register hole or register key. What really happens is that, e.g., for symmetric boundary conditions, overblowing produces a set of frequencies 2fo, 4fo, 6fo, ... The odd multiples of fo are eliminated completely. The period of the sound is now 1/(2fo), and the musical pitch corresponds to 2fo. The change in pitch is discontinuous as a function of how you blow. A register hole or register key makes the instrument easier to play fluently, but you can overblow without needing to use the register hole/key. Why is this? It seems like the effect must be some nonlinearity in the system. Is it a mouthpiece/reed effect, or is it an effect that happens because of the behavior of the air column, tone holes, radiation patterns from the tone holes and bell, ... ? If it's a complicated, nonlinear mouthpiece/reed effect, are there any examples that are easy to understand?