When two straight
lines intersect angles directly opposite
each other, are equal and they are called opposite or vertical angles.

In the quadrilateral ABCD, the angle at A and the angle at C are
said to be opposite angles, as are the angles at B and D.

In all
parallelograms both pairs of opposite angles are equal. In a kite
the two opposite angles on either side of the line of symmetry are
equal.