The 180 Degree Rule

The 180° 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. If the camera passes over the imaginary axis connecting the two subjects, it is called crossing the line. The new shot, from the opposite side, is known as a reverse angle.

http://en.wikipedia.org/wiki/180_degree_rule