The "resonator" supports the oscillations of air generated at the "mouth" of the pipe, a horizontal opening at the juncture of the resonator with the foot. The voicing, the length of the resonator, and the resonator's volume all determine the fundamental pitch of the flue pipe. The conical taper of the pipe determines the overblown pitch. If the pipe is metal, a "tuning sleeve" or "tuning collar" may be fixed at the top of the resonator and raised or lowered to vary its length, thereby adjusting the pitch produced.