Starrett C183 Steel Protractor with Rectangular Head Double graduations from 0-180 degree in opposite directions permitting the direct reading of angles and supplementary angles The back of the tool is flat for ease of use The blade can be locked firmly at any angle by the lock nut Satin chrome finish for ease of reading and resistance to rust Rectangular Head Double graduations from 0 - 180 in opposite directions The back of the tool is flat for ease of use The blade can be locked firmly at any angle by the lock nut Satin chrome finish for ease of reading Rectangular head provides four convenient working edges.