Peg dope
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"Peg dope" (also peg paste, peg stick, peg compound) is a waxy substance which is used to coat the bearing surfaces of the tuning pegs of string instruments (e.g. violins, cellos, violas). It is sold in a small stick (like lipstick), a block (like rosin), or as a liquid in a bottle. Its main function is to increase friction between peg and pegbox - to make sure the pegs don't slip under the pressure of string tension.
When pegs slip, tuning can become virtually impossible. Afflicted violins can be tuned, but minutes later the instrument is effectively unplayable.