More commonly the set square bears the markings of a ruler and a half circle protractor. The outer edges are typically bevelled. These set squares come in two usual forms, both right triangles: one with 90-45-45 degree angles, the other with 30-60-90 degree angles.

Why is it called set square?

It could also be that the "set square" normally comes from a "geometry set" (a set of rulers/tools), so it is the "set square" the "square-making tool from the set".

What is the shape of set square?

Set squares, in the shape of right-angled isosceles triangles, are more than just a common tool in geometry class: they are also frequently used in the field of technical drawing.