Also called co-interior or same-side interior angles. On two parallel lines cut by a transversal, the two interior angles on the *same* side of the transversal don't match — instead they add up to 180°. They make a C-shape (or U-shape).
Drag the ray
110°
obtuse angle
supplement = 70°
The C-rule
- Both angles are between the parallel lines.
- Both on the same side of the transversal.
- They are supplementary — they sum to 180°.
- Conversely, if a pair of co-interior angles sums to 180°, the lines are parallel.
Your turn
Two co-interior angles on parallel lines: one is 110°. Find the other.
Watch out
Easy to mix up: alternate interior angles (opposite sides, Z-shape) are *equal*; co-interior angles (same side, C-shape) *add to 180°*. Different sides → different rule.