Protractor Protractor A protractor is a measuring instrument, typically made of transparent plastic or glass, for measuring angles. Most protractors measure angles in degrees (°). Radian-scale protractors measure angles in radians. Most protractors are divided into 180 equal parts. Some precision protractors further divide degrees into arcminutes. They are used for a variety of mechanical and engineering-related applications. One common use is in geometry lessons in schools. Some protractors are simple half-discs. More advanced protractors, such as the bevel protractor, have one or two swinging arms, which can be used to help measure the angle. A bevel protractor is a graduated
Incidence (geometry) Incidence (geometry) In geometry, an incidence relation is a heterogeneous relation that captures the idea being expressed when phrases such as "a point "lies on" a line" or "a line is "contained in" a plane" are used. The most basic incidence relation is that between a point, , and a line, , sometimes denoted . If the pair is called a "flag". There are many expressions used in common language to describe incidence (for example, a line "passes through" a point, a point "lies in" a plane, etc.) but the term "incidence" is preferred because it does not have the