It Consists of a small metal frame Supported by three legs fixed at the corners of an equilateral triangle A screw of fine pitch passes through the Center of the metal frame The Screw forms the fourth leg The main scale (or pitch scale) is marked on a metal strip fixed at right angles to the frame This scale is marked in millimeters with zero mark at the center (10-0-10 mm) A Circular scale is fixed to the screw head.