If a ray stands on a line, then the sum of the two adjacent angles so formed is 180 degree and the converse. If two lines intersect, vertically opposite angles are equal. Results on corresponding angles, alternate angles, interior angles when a transversal intersects two parallel lines. Lines which are parallel to a given line are parallel. If a side of a triangle is produced, the exterior angle so formed is equal to the sum of the two interior opposite angles.
