The 180 degree rule

The 180° rule is a basic guideline in film making that states that two characters (or other elements) in the same scene should always have the same left/right relationship to each other. If the camera passes over the imaginary axis connecting the two subjects, it is called crossing the line. If the camera crosses the line, confusion results for the film viewers, because it makes it look like the people are switching places as one watches the film.