Crossing the Line

 
  This is one rule that is rarely ever broken by filmmakers. One of the most common shots in films is a conversation between two or more people. Whenever a camera angle changes the same person must remain on the same side of the screen So as not to disorient the viewer - this is known as not crossing the line.
 

In the above example, from camera angle one the green dot is on the left of the screen. Switching to the reverse angle, camera two, the green dot is still on the left. However if we crossed the line and used camera angle 3 the green dot would suddenly be on the right of the screen, exactly where the blue dot used to be. Since two people can't be in the same spot the viewer becomes confused.

The following sequence of clips shows how while the camera angle changes many times during the scene it does not cross the line as each character stays on the same side of the screen.