In the figure above, ∠ABC is a straight angle. Rays BA and BC, which form the sides of ∠ABC, lie in opposite directions. Together, rays BA and BC form a line AC.
Linear pairs and straight angles
A linear pair is a pair of adjacent angles whose non-adjacent sides form a straight angle.
In the diagram below, ∠ABD and ∠CBD form a linear pair. ∠ABD + ∠CBD = ∠ABC = 180°.
Linear pairs aren't the only type of angles that can form a straight angle. A straight angle can be formed as long as the sum of the angles is 180°:
The three angles above share a common vertex, Y, and have a sum of 32° + 100° + 48° = 180°.Thus, ∠XYZ is a straight angle.