When two straight lines cross over each other, four angles are produced at the intersection. These angles form pairs with specific properties:



  • The angles marked 'a' are equal, as are the angles marked 'b'. The pairs of angles 'a' and 'b' are known as "opposite" angles.
  • Pairs of angles 'a+b' are angles on a straight line and will add up to 180°
  • The total of all 4 angles 'a' 'a' 'b' and 'b' comes to 360°