Set square

A set square or triangle (American English) is an object used in engineering and technical drawing, with the aim of providing a straightedge at a right angle or other particular planar angle to a baseline.

triangle, set square

The simplest form of set square is a triangular piece of transparent plastic (or formerly of polished wood) with the centre removed. 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. Combining the two forms by placing the hypotenuses together will also yield 15° and 75° angles. They are often purchased in packs with protractors and compasses.

Less commonly found is the adjustable set square. Here, the body of the object is cut in half and rejoined with a hinge marked with angles. Adjustment to the marked angle will produce any desired angle up to a maximum of 180°.

Geodreieck with marks for engineer projection (see axonometry)

In many European countries, the most common form of set square combines a 90-45-45 triangle with a protractor into a single tool made of clear plastic. Named "Geodreieck" (German colloquial short form of "Geometrie-Dreieick", meaning "geometry triangle"), it was originally developed and trademarked by the German-Austrian manufacturer Aristo-Instrumente Dennert KG (now GEOtec Zeichen- und Kunststofftechnik GmbH) in 1964.[1]