perpendicular lines

Perpendicular lines meet at a right angle (90°), like the corner of a sheet of paper or the cross of a plus sign. Where they cross, they make a perfect square corner.

On the coordinate plane, two non-vertical lines are perpendicular exactly when their slopes are negative reciprocals: if one has slope m, the other has slope −1/m. Equivalently, the product of their slopes is −1. Tilt a line one way and its perpendicular tilts the opposite way and exactly as much, so the steeper one of the pair is mirrored by a shallower one.

The negative-reciprocal rule has one exception: a horizontal line (slope 0) and a vertical line (undefined slope) are perpendicular, even though you cannot multiply their slopes — 0 has no reciprocal. Treat that pair as a special case.