Also known as a "Douglas" protractor. This protractor/plotter has been around a long time in one form or another. It is best suited for rapid plotting of courses and bearings in a confined space. Those who have used these for awhile, swear by them. The 5 inch size is small and inexpensive enough that you should keep one as a spare at the nav station. The large 10 inch size is for full sized nav stations.