The 180° Degree Rule

The 180° Degree Rule:
The 180° degree rule is a guideline that film makers follow and it states that two characters in the same scene should ALWAYS have the same left/right relationship to one another. If the camera passes over an imaginary line connecting the two actors/actresses then this is called crossing the line. The new shit from the opposite side is called the reverse angle.

                   

This picture shows the imaginary axis between the two subjects and the 180° green arc on which the cameras may be placed. When cutting from the green arc to the red arc, the characters switch places on the screen. 

This video shows the rule in more depth and visually how the rule can be put into place. She describes the rule in an example of a football match, as the camera will never switch sides because this will confuse the viewers as they wont know which way their team are attacking or defending.