One line laid across two
A zebra crossing has parallel white bars. A slanted line cuts across all of them.
Step 1 of 4
- Two parallel lines, cut by one transversal.
- Corresponding pair: same corner at each crossing.
- Alternate pair: both inside, opposite sides of the transversal.
- Co-interior pair: both inside, the same side.
The rule
A line that cuts two other lines is a transversal. Two crossings, four angles each. So a transversal makes eight angles in all. Vertically opposite angles at a crossing are equal. So there are at most four different sizes. The four angles between the two lines are the inside ones.
| Name | Where the two sit | Looks like |
|---|---|---|
| corresponding | same corner, each crossing | letter F |
| alternate | inside, opposite sides | letter Z |
| co-interior (interior, same side) | inside, same side | letter C |
The letters only help you spot a pair fast. They do not say if the pair is equal.
Why this matters
Eight angles are a lot to remember. But there are only three names, not eight. Each name tells you where to look. A name is not a fact to memorise on its own.
Two angles are shaded. Tap the name of the pair.
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