Two Lines Cross

When two straight lines cross, they make four angles at the crossing point. Each angle has two neighbours and one angle facing it across the point.

Neighbours, like ∠1 and ∠2, share an arm, and their outside arms lie along one line. They are a linear pair, so they add up to 180°. Opposite angles, like ∠1 and ∠3, touch only at the point: their arms point in exactly opposite directions. These are vertical angles. So are ∠2 and ∠4.

Say the pairs by their arms, not by where they sit on the page. A crossing can be turned any way, and the opposite angle is still the one whose arms continue the first angle's arms through the point.