Perpendicular lines or planes intersect at a right angle (90ยฐ).
In coordinate geometry, two lines with slopes mโ and mโ are perpendicular if mโ ร mโ = -1 (their slopes are negative reciprocals). A vertical line is perpendicular to a horizontal line. The perpendicular bisector of a segment is a line perpendicular to the segment through its midpoint. Perpendicularity is used in construction, architecture, navigation, and defining coordinate axes.
The concept of perpendicularity dates back to ancient Egypt, where builders used ropes with knots to create right angles for pyramid construction. Euclid defined perpendicular lines in his "Elements" (300 BCE). The term "perpendicular" comes from Latin "perpendicularum" meaning "plumb line," used by ancient builders to create vertical lines.