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. A clarinetist moves between the chalumeau and clarion registers through use of the register key; clarinetists call the change from chalumeau register to clarion 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. This produces a note a twelfth above the original note.