Most instruments overblow at two times the speed of the fundamental frequency (the octave), but as the clarinet acts as a closed pipe system, the reed cannot vibrate at twice its original speed because it would be creating a 'puff' of air at the time the previous 'puff' is returning as a rarefaction. This means it cannot be reinforced and so would die away. 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, a perfect twelfth plus a major sixth, above the fundamentals). The clarinet is therefore said to overblow at the twelfth and, when moving to the altissimo register, seventeenth.