Pro Art Protractor 180 Degree 4" Smoke- 0.80 thick, topaz tinted plastic which provide contrast and easy visibility. High quality protractor with beveled edges. Open center with a 4" ruler along the bottom edge. Resistant to breakage and will not discolor. Imprints have sharply defined graduations in one degree increments. Used by engineers, architects, draftspersons, and in schools for geometry instruction.