A tuning peg is used to hold a string in the pegbox of a stringed instrument. It may be made of ebony, rosewood, boxwood or other material. Some tuning pegs are ornamented with shell, metal, or plastic inlays, beads (pips) or rings.
A friction peg is simply a peg around which a string is wound. The tension of the string is controlled by turning the peg, and the peg is held in place by friction in its hole. This contrasts with tuning machines which are more complex devices that use two or more gears to control the tension of the string. Among modern western musical instruments, friction pegs are most often used on violin family instruments, though usually the double bass uses tuning machines. Friction pegs are also used on older instruments, such as the Bulgarian gadulka and the hurdy-gurdy, as well as on flamenco guitars. They are also common on classical Indian instruments such as the sitar, the Saraswati veena, and the sarod, but some like the esraj and Mohan veena often use modern tuning machines instead.
A properly working peg will turn easily and hold reliably, that is, it will neither stick nor slip. Modern pegs for violin and viola have conical shafts, turned to a 1:30 taper, changing in diameter by 1 mm over a distance of 30 mm. (Modern cello pegs have a slightly more aggressive 1:25 taper. 19th century and earlier pegs, for use with gut strings, typically had an even steeper taper of 1:20.)
The taper allows the peg to turn more easily when pulled out slightly, and to hold firmly when pushed in while being turned. Since the typical wear pattern on a peg shaft interferes with this action, pegs occasionally require refitting, a specialized job which amounts to reshaping both pegs and holes to a smooth circular conical taper.
"Peg dope" (also peg paste, peg stick, peg compound) is a substance used to coat the bearing surfaces of the tuning pegs of string instruments (mainly violins, violas, cellos, viols and lutes ). Manufactured varieties are generally sold in either a small stick (resembling lipstick), a block, or as a liquid in a bottle. Commonly used home expedient treatments may include soap, graphite, or talc.
Peg dope serves two different (and almost conflicting) purposes. It both lubricates the peg shaft so it turns easily in the pegbox and provides friction to keep the pegs from slipping with the force of string tension. Tuning pegs that are well fitted and properly doped will both turn smoothly throughout an entire rotation and hold firmly wherever the player wishes.
Without the proper amount of friction to hold the peg in place, a tuning peg will tend to "slip", making a tuning setting virtually impossible to maintain. String instruments with pegs that are slipping can be tuned briefly, but will be out of tune within minutes as soon as the peg slips again. With too much friction, adjusting the tuning at all is impossible. If the pegs or their holes are not perfectly round, or if the bearing surfaces of the pegs are indented from wear, peg dope will not remedy the resulting problems.
Pegs for double bass and guitar family instruments are usually geared, and are called tuning machines or machine heads. Geared pegs for violin family instruments also exist, although they have not gained wide use, which has to do with the extensive and irreversible physical modification that must be made to the peg box in order to mount them, which is often viewed as ruining the aesthetics of the instrument, combined with a bad reputation they acquired due to poorly designed early models that were prone to failure, often with catastrophically damaging results.
The most recently marketed pegs of this sort use planetary gears designed to fit inside a case shaped like a friction peg, and have seen some adoption as they require no more modification of the instrument than a new set of friction pegs and make fine tuners unnecessary.
Pegs are tapered, and must contact the insides of the holes on both sides of the pegbox. The fit is very important, of course, as is the nature of the contacting surfaces there. Raw wood on raw wood never works well, so a preparation is usually applied to facilitate a smooth turning of the peg.