180 Degree Rule - Charlotte Woodward & Kim Bradbury

Deffinition: The 180° rule is a basic guideline in film making that states that two characters 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. - (wikapedia).