180 Degree Rule

The 180 degree rule is vital in film making as it helps identify 2 characters (or other subjects) in the same scene and should always have a left or right relationship with each other.If the camera passes over this imaginary 180 degree axis that is linking the two subjects, it confuses the audience as the subject has swapped positions, this is unnatural to the human eye due to the organised positioning of the characters in earlier shots . This is called crossing the line. The shot that is now created from the opposite side is called the reverse angle. The crossing of the line can easily be corrected with the buffer shot of the subject head on or from behind, so that the flowing movement of the camera appears to be more subtle and goes unnoticed. Sometimes the crossing of the line can be deliberate to cause this disorientation "The Shining" by Stanley Kubrick is an example of this.

Here is a simplistic diagram of the 180 degree rule taken from Wikipedia:


when the characters are transferred
from the red arc to the green arc, it switches their
positions on the screen.

here is also a video to explain the 180 degree rule further:
http://www.youtube.com/watch?v=BBPw9C57TuU