A pair of angles is sai to be vertical (US English) or opposite (British English) if the angles share the same vertex and are bounded by the same pair of lines but are opposite to each other. Such angles are congruent and thus have equal measure. If two line segments, EF and GH, intersect at the point P, they form four angles, EPG, GPF, FPH, and HPE. These angles can be grouped into two pairs of vertical angles: one vertical pair contains EPG and FPH, and the other pair contains GPF and HPE. Any angle in the first pair is supplementary to any angle in the second pair. The most obvious way to tell if two angles are vertical angles are if they form a "X"