Created where a
transversal
crosses two (usually parallel) lines.
Each pair of interior angles are inside the parallel lines, and on the same side of the transversal.

Try this Drag an orange dot at A or B. Notice that the two interior angles shown are
supplementary (add to 180°) if the lines PQ and RS are parallel.

Referring to the figure above, the
transversal AB crosses the
two lines PQ and RS, creating intersections at E and F.
Each pair of interior angles are inside the parallel lines and on the same side of the transversal.
There are thus two pairs of these angles. In the figure above, click on 'Other angle pair' to show each pair of interior angles in turn.

Remember: __in__terior means __in__side the parallel lines.

If the transversal
cuts across parallel lines (the usual case) then the **interior angles are
supplementary (add to 180°)**.
So in the figure above, as you move points A or B, the two interior angles shown always add to 180°.
Try it and convince yourself this is true. Click on 'Other angle pair' to visit both pairs of interior angles in turn.

If the transversal
cuts across lines that are not parallel, the interior angles still add up to a constant angle, but the sum is not 180°.

Drag point P or Q to make the lines non-parallel. As you move A or B, you will see that the interior
angles add to a constant, but the sum is not 180°.
(The angles are rounded off to the nearest degree for clarity, so bear that in mind if you check this).

- Corresponding angles
- Alternate interior angles
- Alternate exterior angles
- Interior angles of a transversal
- Exterior angles of a transversal

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All rights reserved