Explore parallel lines

Expand the section below. Are the long lines parallel?

An optical illusion involving parallel lines

Let’s learn about angles on parallel lines.

A transversal is a line that cuts two or more lines. When a transversal cuts two or more lines, some special angles are formed.

Co-interior angles

Here’s the lowdown: If the arms of two angles form a shape looking like the letter C or U, those angles are called co-interior angles. The C or U could be backwards or upside-down, and it might look a bit wonky.

Have a look at this co-interior angles interactive.

What do you notice about co-interior angles on parallel lines?

Alternate angles

Here’s the lowdown: If the arms of two angles form a shape looking like the letter Z or N, those angles are called alternate angles. The Z or N could be backwards or upside-down.

Have a look at this alternate angle interactive.

What do you notice about alternate angles on parallel lines?

Corresponding angles

Here’s the lowdown: If the arms of two angles form a shape looking like the letter F, those angles are called corresponding angles. The F could be backwards or upside-down.

Have a look at this corresponding angles interactive.

What do you notice about corresponding angles on parallel lines?

So what’s so special about these angles?

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Open this interactive page and explore it by following these steps:

Find the pairs of corresponding angles, alternate angles and co-interior angles.
Move the blue points to make the lines parallel.

What happens to pairs of alternate angles as you move from non-parallel to parallel lines?
What happens to pairs of corresponding angles as you move from non-parallel to parallel lines?
What happens to pairs of co-interior angles as you move from non-parallel to parallel lines?

Explore parallel lines

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