A wooden set square from the 1930s-40s.
In classical geometry, a set square is an instrument used to draw right angles and, with a straight edge particularly, parallel lines.
The most basic form of set square is a triangular piece of clear plastic or wood with the center removed and the outer edges beveled. Commonly, the markings of a ruler and a half circle protractor are inscribed. These set squares come in two usual forms, with 90-45-45 degree angles and 30-60-90 degree angles. Combining the two forms by placing the hypotenuses (the longest sides) together yields 15° and 75° angles.