Opposite Angles

Part of the Polygons & Quadrilaterals section of Coddy's Geometry journey. Lesson 16 of 59.

Go round a parallelogram one corner at a time. With ∠A = 70°, the neighbour ∠B is 110°. The next neighbour, ∠C, is 180 − 110, which is 70° again, and ∠D is 110°.

$Parallelogram with angles 70, 110, 70 and 110 degrees$

So opposite angles of a parallelogram are equal. They are two steps apart round the shape, and each step takes 180° minus the angle before. The four angles still add up to 360°: 70 + 110 + 70 + 110.

The board has no single rule for opposite angles. Go round the corner instead: two steps of Co-interior angles, one along each side.