The 180 Degree Rule

In film-making the 180° rule is a basic guideline that states that two characters in the same scene should always have the same left and right relationship to each other. If the camera passes over the imaginary line connecting the two characters, it is called crossing the line. If you do this you have to show it. The new shot, from the opposite side, is known as a reverse angle.

In the example of a dialogue between two actors, if actor 1 (orange shirt in the diagram) is on the left and actor 2 (blue shirt) is on the right, then actor 1 should be facing right at all times, even when actor 2 is off the edge of the frame, and actor 2 should always be facing left. Shifting to the other side of the characters on a cut, so that actor 2 is now on the left side and actor 1 is on the right, will disorient the viewer, and break the flow of the scene.