Trigonometry Trigonometry is based on the study of right-angled triangles. This is a form of geometry that developed out of the study of the stars. It is also concerned with the relationships that exist between the sides and angles of triangles, and their measurement. Trigonometry comes in two forms, spherical and plane. Spherical is the study of curved triangles lying on the surface of a hypothetical sphere, which is mainly used by astronomers and navigators. Plane is the study of more variety and is mainly focused on by many books on trigonometry. In trigonometry, the ratio between any two sides of a r