Used to ascertain the correct angle while hanging hammers. The shanks must have been traveled first, as the fact that they are moving vertically toward the strings is the basis for the square of the hammer. The glued hammer is held at its approximate string contact position, and the jig is placed so that the wires are equidistant to the sides of the hammer molding at the shank (spaces are equal on each side. Actually, what you want is for the wires to be equidistant from the center of the shank). The hammer is then dropped to rest position, and twisted as needed so that the crown is centered between the wires.