The 24 Inch Parallel Rule is primarily used for commercial purposes due to its extra long 24 inch acrylic construction, rigid chrome arms and handles. Parallel rulers are used to plot direction on nautical charts. Two straightedges are hinged so that they maintain the same angle. By alternating the moving edge, and securely holding down the nonmoving edge, you can move the rulers about the chart while still maintaining the same angle. Milled acrylic with bevelled edges Includes protractor scale Lifetime warranty