"Peg dope" (also peg paste, peg stick, peg compound) is a substance used to coat the bearing surfaces[1] of the tuning pegs of string instruments (mainly violins, cellos, violas). Manufactured varieties are generally sold in either a small stick (resembling lipstick), 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 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.