0 one of two equal angles on opposite sides of a line that crosses two usually parallel lines (= lines that are always the same distance apart), and on opposite sides of those lines -- 錯角，交錯角（交錯角是兩個相等的角之一：平行的兩條線被第三條直線所截，這第三條直線的兩側就形成兩個相等的交錯角，兩條平行線的兩側也各形成兩個交錯角）
Then one of the alternate angles is an exterior angle equal to the other angle which is an opposite interior angle in the triangle.
Two lines are parallel if and only if the two angles of any pair of alternate angles of any transversal are congruent (equal in measure).
