The 180-degree rule of shooting and editing keeps the camera on one side of the action. As a matter of convention, the camera stays on one side of the axis of action throughout a scene; this keeps characters grounded compositionally on a particular side of the screen or frame, and keeps them looking at one another when only one character is seen onscreen at a time. The technique allows for an expansion of the frame into the unseen space offscreen. It is referred to as a rule because the camera, when shooting two actors, must not cross over the axis of action; if it does, it risks giving the impression that the actors' positions in the scene have been reversed.
There are instances when the 180-degree rule is violated. For instance, the director Yasujro Ozu often tampers with sight lines and crosses the axis with ease. Ozu reverses camera angles, breaking with convention, and creates an almost purely cinematic tension within scenes. We are forced to see characters with reversed screen placement, sight lines, and even movement. While the continuity of the scene holds, this change of placement serves to make the audience both uneasy and attentive.