The Bi-rola Rule is a rugged plotter that is used by professional navigators because it is quick and easy to use. The patented vertical and horizontal rolling motion simplifies course and plotting calculations. - Adjustable radial arm allows automatic compass corrections in course plotting- Roller body with protractor markings & nautical mile scale- Radial arm has Loran interpolator scales & 200 millimeter rule- When radial arm & body are used jointly, the Bi-Rola becomes a 17 inch divider- Speed/ time/ distance table included- Easy to follow instructions on back of protective, durable vinyl pouch- Lifetime warranty