The lower or 'chalumeau' register plays fundamentals, whereas the upper or 'clarino' register, aided by the register key, plays third harmonics, a perfect twelfth higher than the fundamentals. The altissimo range, aided by the register key and venting with the first left-hand hole, plays 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 (the Rackett or Sausage Bassoon is the next most common Western instrument that overblows at the twelfth). 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.