A lathe is a machine tool used to shape components whose cross-sections are circular, but whose diameter usually varies along their length. Typically, the workpiece is held in a stock or lathe bed, and supported and rotated by components at each end, the headstock and tailstock. Cutting tools are introduced from the side to shape the part. In a screw-cutting lathe, the tool is moved along the piece to produce a thread of given depth and pitch. Modern lathes may hold several workpieces and use several cutting tools.