The Bi-rola™ represents Weems & Plath's effort to provide the very best plotting tools. Constructed of heavy duty plastic with brass rollers, the Bi-rola™ rule is built to withstand years of trouble free use. The Bi-rola™ rule's patented vertical and horizontal rolling motion simplifies course and plotting calculations, while the unique radial arm allows you to automatically include compass correction in your course plotting. A speed/time/distance table and vinyl pouch is included. Dimensions: length 13-7/8", width 2-5/8", thickness 1".