Abstract: A three-roll thread gauge for measuring interior threads includes a fixed gauge roll and two movable gauge rolls. The motion of the two movable gauge rolls is constrained so that the interior angles of a triangle defined by the three gauge rolls are equal for each position of the first and second gauge rolls when the three gauge rolls are in engagement with an interior thread. The motion is constrained by reverse sine bars that limit the motion of first and second tables to which the first and second gauge rolls are attached.