middle D, and so forth. Once the tuning stop is fully in tune with itself, the rest of the stops are tuned. Most stops are tuned to the tuning stop, though some stops are more easily tuned to stops other than a 4′ Principal. Tools and Techniques The most common tuning tool is called a "tuning knife". It is a piece of metal used to tap gently on the tuning mechanism of a pipe, so as to avoid touching the pipe with the hands. The techniques for tuning flue pipes vary with the construction of the pipe: An open metal pipe usually has a sliding collar ("tuning slide") at the top of the pipe that can be moved to change the pitch. An open wooden pipe may have a metal flap partially covering its top, which can be rolled or unrolled, or bent upward or downward. On a slotted metal pipe, some or all of the metal cut out to make the slot is rolled up so the slot can effectively be shortened or lengthened, thus changing the pitch of the pipe. On a slotted wooden pipe, a wooden slider is provided to shorten or lengthen the slot. A stopped pipe (wood or metal) is usually tuned by moving its stopper up or down. A capped pipe is usually tuned by moving its cap up or down. A conical metal pipe will sometimes have a tuning slide, but often is tuned by moving the large ears on either side of the pipe's mouth. Small metal pipes are often "cone tuned", whereby the top of each pipe is deformed inward or outward using a heavy hollow cone. Such tuning is extremely stable, but causes gradual damage to the pipe over time. Reed pipes may be tuned in any of several ways: (1) by lengthening or shortening the vibrating length of the reed tongue by means of a wire protruding from the boot of the pipe; (2) by adjusting the effective speaking length of the resonator; (3) by adjusting the metal flap in the side of the resonator or the cap on the top of the pipe (especially with fractional length pipes). All of these methods can also affect the tonal regulation of the pipe, so tuning reed pipes is trickier than tuning flue pipes. Miscellaneous Organ pipes are so sensitive to temperature that the body heat of the organ tuner can