Parallel lines are indicated by arrows going in the same direction. Parallel lines are indicated by arrows going in the same direction. If there is more than one set of parallel lines in a diagram, ...

A collaborative online mathematics project holds lessons for other disciplines. Crowd-sourcing has reached mathematics, and at first glance it might seem as if this stereotypically solitary discipline ...

Horizontal lines run left to right, parallel to the horizon, with slope 0 and equation y = k. Vertical lines run top to bottom, perpendicular to the horizon, with undefined slope and equation x = k.