# Angles in parallel lines

When a line crosses two parallel lines, matching angles are equal. Alternate angles form a Z shape and are equal. Co-interior angles form a C shape and add to 180 degrees.

Maths (school) · Part 05: Angles and area · Card 2 of 8 · Last updated 2026-10-19

## Analogy

Like railway tracks crossed by a road: the same angle repeats at each crossing.

## Example

**Z-shape and C-shape**

Alternate angle = 62, so the other = 62
Co-interior angle = 62
The partner = 180 - 62 = 118

## In practice

Road designers and carpenters use these rules to keep lines truly parallel.

## Practice

### Flashcards

- **Name the angle pair that forms a Z shape in parallel lines.** Alternate angles. They are equal.

## Questions people ask

### How do you spot alternate angles?

Look for a Z shape (or an S shape) made by the crossing line and the two parallel lines. The angles tucked inside the corners of the Z are alternate angles, and they are equal. Always check the lines are parallel, usually shown by arrows.

### What is the difference between corresponding and co-interior angles?

Corresponding angles sit in the same position at each crossing, like an F shape, and are equal. Co-interior angles sit between the parallel lines on the same side, like a C shape, and add up to 180 degrees instead of being equal.

## Related concepts

- [Angle rules](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-1)
- [Angles in polygons](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-3)
- [Circle theorems](https://studyvisually.com/learn/science/maths-school/maths-school-part-5/maths-school-part-5-concept-8)

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