Screw gauge works on the rule of screw. It has a linear scale called as the main scale, and another scale called as the circular scale. The circular scale can be moved by a head screw. On moving the screw, the circular scale advances linearly on the major scale. The distance covered by the tip of screw when it is given single complete movement, is called as the pitch of the screw. Split the pitch of screw by the total number of division on the circular scale, we attain the distance which the screw advances on rotating the screw by 1 division on its circular scale. This distance is called as the least count of the measurement device.