'The 180-degree rule' is a basic guideline in the film making industry that regards the on-screen spatial relationship between one character and another character or even an object within a scene. An imaginary line referred to as the axis connects the characters, and by keeping the camera on one side of this axis for every shot in the scene, the first character is always frame right of the second character, who is always then frame left of the first. If the camera passes over the axis it is called jumping the line or crossing the line.
For example, in a dialogue scene between two characters, Daniel (orange shirt, frame left in the diagram) and Lucas (blue shirt, frame right), the camera may be placed anywhere on the green 180° arc and the spatial relationship between the two characters will be consistent from shot to shot, even when one of the characters is not on screen. Shifting to the other side of the characters on a cut, so that Lucas is now on the left side and Daniel is on the right, may disorient the audience.
(http://en.wikipedia.org/wiki/180-degree_rule)
