180 Degree Rule

In filmmaking, the 180 degree rule is a basic 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.



Above is a basic picture that helps me to understand the rule.