The angles opposite each other when two lines cross. They are always equal.

In this example a° and b° are vertical angles.

"Vertical" refers to the vertex (where they cross), NOT up/down. They are also called vertically opposite angles.

Try moving the points below. Notice that the 4 angles are actually two pairs of vertical angles:

In this example a° and b° are vertical angles.

"Vertical" refers to the vertex (where they cross), NOT up/down. They are also called vertically opposite angles.

Try moving the points below. Notice that the 4 angles are actually two pairs of vertical angles:

See: Vertex

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