The 180 degree rule is a basic film editing guideline that states that two characters (or other elements) in the same scene, should always have the same left/right relationship to each other. In the example of a dialogue, if A is on the left and B is on the right, then A should be facing right at all times, even when B is off the edge of the frame, and B should always be facing left. Shifting to the other side of the characters on a cut, so that B is now on the left side and A is on the right, will disorient the viewer, and break the flow of the scene.

The rule gets its name from the 180 degree arc that extends from a point on the other side of A from B, to a point on the other side of B from A, within which A and B will always have the same left/right orientation.

The 180 degree rule is also often called "The Line", or the "Line of Action". An imaginary line can be drawn between two characters, and the camera should stay on one side of this line. Breaking this rule can be referred to as "breaking the line of action", or "breaking the 180 degree line".

The 180 degree rule is an essential element of a style of film editing called continuity editing. The rule is not always obeyed; sometimes a filmmaker will purposefully break the line of action in order to create disorrientation.