Perpendicular lines

In plane geometry, two lines are perpendicular if they intersect to form four right angles (each measuring 90°). If two lines are perpendicular, each of the angles formed at the intersection is a right angle.

Important properties

• If two lines are perpendicular, the product of their slopes is -1 (in the coordinate plane).

• Perpendicularity is a symmetric relation: if line $l$ is perpendicular to line $m$, then $m$ is perpendicular to $l$.

• Perpendicular lines divide the plane into four right angles at the intersection point.