Wiktionary
capstan screw
n. 1 A type of screw commonly used to regulate the height of the action in a piano. The capstan screw is designed to be turned from the side, not the head, unlike an ordinary screw. A lever enters a hole in the side of the screw to turn it. 2 (context nautical English) Mainly on sailboats, the mechanism for winding ropes under a lot of tension; a capstan.