180-degree rule

Keep the camera on one side of the action's axis, and screen directions stay truthful. Cross the line and the world flips.

The rule draws an invisible line through the scene, between two speakers, along a chase, and asks every setup to stay on one side of it. Rewarded: he looks left, she looks right, geography holds across any number of cuts. Crossed carelessly: the same pair suddenly face the same way, and the room silently breaks.

Crossing is legal with paperwork: a moving shot that carries the audience over the line, a neutral frame on the axis, a new scene. And crossing raw is a weapon, fights and breakdowns cut across the line precisely to disorient. The rule is not etiquette; it is the audience's map, and maps are redrawn on purpose or not at all.