Bennett, J. and Johnston, S., The Geometry of War, 1500-1750 (Oxford, 1996)
72. A German protractor with a semicircle graduated in degrees and marked with positions for regular polygons of between four and twenty sides. Apart from '4', at the centre (90 degrees), representing the square, each polygon has two marks distributed symmetrically about the right-angle position, so that the marks run from 20 to 4 to 20. These positions give both the interior angles between the walls and the complementary angles subtended by the walls at the centre of the polygon.
Diameter: 100 mm
Billmeir Collection
Inventory no. 32,196