Co-Interior Angles

What are co-interior angles?
Co-interior angles are a pair of angles that are formed when a transversal line intersects two other lines. These angles lie on the same side of the transversal and are located between the two intersected lines. They are also sometimes referred to as consecutive interior angles or same-side interior angles.

The co-interior angles are the two interior angles that are on the same side of the transversal. They often form a sort of “C” shape.

What is the Consecutive Interior Angles Theorem?
The Consecutive Interior Angles Theorem states that if two parallel lines are cut by a transversal, then consecutive interior angles are supplementary (= 180°).

The converse of this property is also true:
If a transversal intersects two lines such that a pair of co-interior angles are supplementary, then the two lines are parallel.

The following figures give an example of co-interior angles and an illustration that co-interior angles are supplementary. Scroll down the page for more examples and solutions.

Geometry Worksheets
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Printable & Online Geometry Worksheets

Understanding co-interior angles is crucial when working with parallel lines and transversals in geometry. They are one of several pairs of angles formed in this situation, including alternate interior angles and corresponding angles, each with their own unique properties when the lines are parallel.

What is the Alternate Interior Angles Theorem?
The Consecutive Interior Angles Theorem states that if two parallel lines are cut by a transversal, then each pair of alternate interior angles is congruent.

How to use the Consecutive Interior Angles Theorem?

Parallel lines : Alternate Interior, Corresponding and Co-interior angles

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Parallel Lines and Interior Angles
Relationships between Parallel Lines and Interior Angles

Parallel Lines and Transversal
Helps describe the angle pairs formed by parallel lines and a transversal. Looks at corresponding, alternate interior and same side interior angles

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