Vertical angles,

vertically opposite angles

Vertical angles are the angles that lie opposite to each other when two lines intersect.

Two vertical angles always share a common vertex. However, they never share a common side (arm).

«Vertical» in this case means they share the same Vertex (corner point), not the usual meaning of up-down.

Example: 

∠AOD and ∠BOC are vertical angles.

∠AOC and ∠BOD are vertical angles.

Vertical angles are congruent (the same number of degrees)

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