It is mounted on an aluminum roller assembly that allows you to plot courses and parallel lines with simple direct transfer and doesn’t require the space that traditional parallel rules or triangles need. The roller keeps the plotter parallel as you slide it across the chart. It includes distance scales at 1:20,000, 1:40,000 and 1:80,000 to fit U.S. Coast and Harbor Charts, as well as an inch scale. A semi-circular protractor for measuring course and bearing against meridian on a chart and two quarter circle protractors for measuring course and bearing against parallel of latitude are built into the plotter. It is also used for measuring or laying off courses and distances on a nautical chart.