Crack, for flute, alto saxophone, and electro-acoustic sounds, is an exploration of multiphonics and other sounds that are possible on these two instruments, yet lie out of the realm of their traditional playing techniques and tone production. During the summer and fall of 2000, I began an intensive study of the harmonic properties of each instrument's set of multiphonics. In an attempt to crack the inner workings of multiphonics, I discovered some intriguing facets of their harmonic spectra, which led to some interesting conclusions on timbral relations. The electro-acoustic sounds are made up of analyses of pre-recorded multiphonics (FFTs) that are resynthesized into slowly changing timbres (acting as resonances of the instrumental music) and reveal a fascinating world within the cracks "between the keys".