Definition of the vertical angles

 

Two angles are called vertical if the sides of one angle are continuations of the sides of the other angle.

 

∠ВОС and ∠AOD are vertical angles as side ОВ of angle ВОС is a continuation of side OD of angle AOD and side ОC of angle ВОС is a continuation of side OA of angle AOD.

∠AOB and ∠DOC are vertical angles as side ОВ of angle AOB  is a continuation of side OD of angle DOC and side ОC of angle DOC is a continuation of side OA of angle AOB.

Definition of the vertical angles

∠ 1 and ∠3 are vertical angles

∠ 2 and ∠ 4 are vertical angles