Vertical (angles)

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A pair of angles are said to be vertical (US English) or opposite (British English) if they share the same vertex and are bounded by the same pair of lines but are opposite to each other. Such angles are vertical (congruent). More precisely, if two lines AB and CD intersect at E, and E is between A and B, and E is between C and D, then angle AEC is vertical to angle BED, and angle AED is vertical to angle BEC.

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Definition and properties of vertical angles With interactive applet
Angle definition pages with interactive applets that are also useful in a classroom setting. Math Open Reference