Glass dials are simply clock glasses on which the hour and minute numerals are marked instead of on a conventional dial. Such dials make it possible to see the movement of the clock in the same manner as in a skeleton clock. Although not common, glass dials are found on some early Connecticut shelf clocks, on a type of keyless rack clock made in the 1920s, and on a famous one-wheel clock invented by Pierre Le Roy in the 18th century. Another form of glass dial is used in a type of 'mystery' clock in which the hands are marked on revolving glass discs, driven from their edges, the clock thus appearing to have no movement.