Useful for both charts and topo maps. Often called a Compass Protractor. Made of clear plastic. Can be used as a Small Craft Navigation Aid (aka course plotter) by drilling a hole through the center point and tying a length of string. 3.5 inch diameter. Flexible plastic. C-Thru Ruler no. 255 protractor. This is the same C-Thru Ruler Product now with the Wescott or ACME name.