The 180 Degree Rule of Film Making

To maintain continuity, draw a line between 2 actors and keep the camera on one side of that line. Only break the line when there is a very compelling reason to. This is one of the basic principals of filmmaking and it’s easily demonstrated during a football game. If Team A is on the left side of the screen and they are trying to score on the right side of the screen, switching to a shot where Team A is rushing into the end zone on the left side of the screen would confuse viewers. The audience would thing Team A just scored on their own end zone! There is another great writeup on the Mark Kennedy’s Site site.