Drawing showing the difference between right-handed and left-handed screw threads. The handedness of screw threads is determined by the right hand grip rule; in a right-handed screw, if the fingers of the right hand are wrapped around the screw in the direction the screw is turning, the thumb will point in the direction the shaft moves. The type of screw thread shown is called a square thread. Alterations to image: cropped screw thread images out of source drawing, rotated them 90° and added captions