The 180 Degree Rule

The 180° rule is a cinematography guideline that states that two characters in a scene should maintain the same left/right relationship to one another . When the camera passes over the invisible axis connecting the two subjects, it is called crossing the line and the shot becomes what is called a reverse angle

When the camera initially establishes where two facing characters are standing E.g. John(left) Mark(right). The 180° rule comes into play as the close up shots must be within the 180° barrier. 

If the camera is outside this barrier then it would look like the characters have switched position, contradictory to the establishing shot. This effects the continuity of the scene as it would make no sense if the characters switched positions.