Designed for use with the William Mitchell Round Hand Dip pens. Select the disc for the appropriate nib size, and choose the edge of the disc that corresponds to the required number of nib widths. Use the ticks as a guide to mark the paper at the correct x-height distance. The marks are now joined and your paper is ruled up at the correct x-height. Because there is no need to measure awkward combinations with a ruler, The X-Height Measuring Discs provide an accurate and faster method of ruling up your paper.