Vertically Opposite Angles occur whenever two lines or line segments intersect. Vertically opposite angles are always equal.[br]In the diagram below, the red angles are vertically opposite, as are the blue angles.

Click and drag the points A and B. Notice that the angles change size but the opposite angles are always equal.[br][br]Notice that the adjacent angles always add to 180 degrees. Such angles are called [b]supplementary angles[/b].