Right Angles — The Geometry of 90°

A right angle is exactly one quarter of a full rotation. Why 360 degrees? The Babylonians divided a circle into 360 because it is highly composite — divisible by 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24. The right angle (90°) defines perpendicularity: two lines meeting at right angles have slopes whose product is −1. Perpendicularity is the foundation of Cartesian coordinates, the Pythagorean theorem, and the inner product in linear algebra. Every building you stand in is constructed around the right angle.