section_path: Main concepts/Angles
section_name: Angles
paragraph_text: Angles
  
  Euclid defines a plane angle as the inclination to each other, in a plane, of two lines which meet each other, and do not lie straight with respect to each other. In modern terms, an angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle. The size of an angle is formalized as an angular measure.
  
  In Euclidean geometry, angles are used to study polygons and triangles, as well as forming an object of study in their own right. The study of the angles of a triangle or of angles in a unit circle forms the basis of trigonometry.
  
  In differential geometry and calculus, the angles between plane curves or space curves or surfaces can be calculated using the derivative.