"Peg dope" is a substance used to coat the bearing surfaces of the tuning peg

Tuning peg

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 or rings....

s of string instrument

String instrument

A string instrument is a musical instrument that produces sound by means of vibrating strings. In the Hornbostel-Sachs scheme of musical instrument classification, used in organology, they are called chordophones...

s (mainly violin

Violin

The violin is a string instrument, usually with four strings tuned in perfect fifths. It is the smallest, highest-pitched member of the violin family of string instruments, which includes the viola and cello....

s, cello

Cello

The cello is a bowed string instrument with four strings tuned in perfect fifths. It is a member of the violin family of musical instruments, which also includes the violin, viola, and double bass. Old forms of the instrument in the Baroque era are baryton and viol .A person who plays a cello is...

s, viola

Viola

The viola is a bowed string instrument. It is the middle voice of the violin family, between the violin and the cello.- Form :The viola is similar in material and construction to the violin. A full-size viola's body is between and longer than the body of a full-size violin , with an average...

s). Manufactured varieties are generally sold in either a small stick (resembling lipstick

Lipstick

Lipstick is a cosmetic product containing pigments, oils, waxes, and emollients that applies color, texture, and protection to the lips. Many varieties of lipstick are known. As with most other types of makeup, lipstick is typically, but not exclusively, worn by women...

), a block, or as a liquid in a bottle. A commonly used home expedient treatment involves soap and chalk, in varying proportion depending on whether the peg tends to slip or stick.

Peg dope serves two different (and almost conflicting) purposes. It both lubricates the peg shaft so it turns easily in the pegbox

Pegbox

A pegbox is the part of certain stringed musical instruments that houses the tuning pegs....

 and provides friction to keep the pegs from slipping with the force of string

Vibrating string

A vibration in a string is a wave. Usually a vibrating string produces a sound whose frequency in most cases is constant. Therefore, since frequency characterizes the pitch, the sound produced is a constant note....

 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.