Angles: Basics – More Key Points

Lines that intersect


When lines intersect, there are certain facts that arise that help us to do calculations:

  • Two lines crossing each other as in our illustration will produce an “X” shape and the following will always hold:
  • The angles that are “opposite” to each other, or more precisely “vertically opposite” are equal in size. So the red angles are equal and the green angles are equal.
  • If we added the value of all the angles in this figure they always add to 360ᵒ


A straight line is said to have an angle of 180ᵒ at any point along it, so if we create two or more angles at a specific point, the total of the angles at that point will always add to 180ᵒ.