1) The 180 degree rule:
This is when a line of action is created due to the interaction of the characters and/or subjects. The camera has to stay one side of the line, and footage has to be taken in the 180 degree arc around the line. This is done to prevent confusion and disorientation in the audience.
If the axis is crossed it is called 'jumping the line' or 'crossing the line' it could stop continuity of the filming. The only time the line is crossed is to create an effect (sense of confusion).
YouTube:http://www.youtube.com/watch?v=HinUychY3sE
slideshare:http://www.slideshare.net/tijoum/the-180-degree-rule
