Covering or uncovering the tone holes varies the length of the pipe, changing the resonant frequencies of the enclosed air column and hence the pitch of the sound.A clarinetist moves between the chalumeau and clarino registers through use of the register key, or speaker key: clarinetists call the change from chalumeau register to clarino register "the break".[The open register key stops the fundamental frequency from being reinforced and the reed is forced to vibrate at three times the speed it was originally vibrating at. This produces a note a 12th above the original note. 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 the original speed because it would be creating a ‘puff’ of air at the time the previous ‘puff’ is returning as a rarefaction. This means that it cannot be reinforced and so would die away.