Abstract: A set square has substantially the form of a 30.degree./60.degree./90.degree. triangle having therein an aperture having sides at 15.degree. and 45.degree. to each external side. The aperture may be in the form of a 30.degree./30.degree./120.degree. triangle disposed with its longest side closest to the hypotenuse of the 30.degree./60.degree./90.degree. triangle and at an angle of 15.degree. thereto, said side diverging from the hypotenuse towards the 30.degree. vertex of the 30.degree./60.degree./90.degree. triangle; preferably a line is indicated perpendicular to one of the external sides other than the hypotenuse, the line being indicated adjacent said side and extending across said aperture, and being indicated on either side thereof. There may be a protractor scale whose origin is at the intersection of the line and the perpendicular side.