Spherical geometry is the study of objects on the surface of a sphere; this differs from the type of geometry studied in plane or solid geometry. In spherical geometry, there are no parallel lines, and straight lines are actually great circles, so any two lines meet in two points. In addition, the angle between two lines is the angle between the planes of the corresponding great circles. There are also entities called spherical triangles (or Euler triangles), when three planes pass through the surface of a sphere and through the sphere’s center of volume; they have three surface angles and three central angles. There are also spherical polygons, in which a closed geometric object on the surface of a sphere is formed by arcs of great circles.