0 one of the two angles on either side of a straight line that divides a larger angle into two -- 鄰角（由角分線將一個大角分成的相鄰兩個小角中的一個）
And they are adjacent angles.
When two adjacent angles form a straight line, they are supplementary.
In a right trapezoid (also called right-angled trapezoid), two adjacent angles are right angles.
The "watari-yagura-mon" was constructed at adjacent angles to each side within the gate.
The proposition showed that since both of a pair of vertical angles is supplementary to both of the adjacent angles, the vertical angles are equal in measure.
More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles.
