 The chalumeau register plays fundamentals, whereas the clarion register, aided by the register key, plays third harmonics, a perfect twelfth higher than the fundamentals. The first several notes of the altissimo range, aided by the register key and venting with the first left-hand hole, play fifth harmonics, a major seventeenth (that is a perfect twelfth plus a major sixth) above the fundamental. The clarinet is therefore said to overblow at the twelfth, and when moving to the altissimo register, a seventeenth. By contrast, nearly all other woodwind instruments overblow at the octave, or like the ocarina and tonette, do not overblow at all ). A clarinet must have holes and keys for nineteen notes (a chromatic octave and a half, from bottom E to B ♭) in its lowest register to play the chromatic scale. This overblowing behavior explains the clarinet's great range and complex fingering system. The fifth and seventh harmonics are also available, sounding a further sixth and fourth (a flat, diminished fifth) higher respectively; these are the notes of the altissimo register. This is also why the inner "waist" measurement is so critical to these harmonic frequencies.
