Parallel lines are a fixed distance apart and will never meet, no matter how long they are extended. Lines that are parallel have the same gradient. The graphs above, \(y = 2x + 1\) and \(y = 2x - 2\) ...

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.

In flat space, two parallel lines never meet, however, they appear to meet at very distances for the same reason due to which objects appear smaller when they are at a distance than when they are ...