Wider reading: 180 degree rule

The 180-degree rule is possibly the most basic principle of filmmaking and production. Whilst using the rule can establish characters and action; resulting in the audiences focused sight line and continuity of the scene, disregarding the rule for dramatic effect can create disorientation for the viewers. 

>To understand the 180-degree rule, you must first understand the definition of 'Lines of action. This consists of an imaginary line between the subject and the item or person that they are interacting with. This may also be conveyed with movement, by which an imaginary line is displayed on a path in which the subject is moving on. Many things can establish lines of action, for example:
 -Camera movement
 -Neutral shots 

 -Cut away's
New action lines are always established through the 180-degree arc, and create shifts within the scene.

>The 180-degree rule can be shown by this diagram. All shots should be filmed from within the 180-degree arc to maintain  screen direction. If shots are outside of this arc or behind the line of action, viewers may be disorientated and confused as their eye-line and focus can change. 

>Directors often start with establishing  shots or master shots to give their audience a basic geography of the scene, this determines screen direction. This is one way that directors are able to break the 180-degree rule as the establishing shot allows them to shift the action line. This implies disorientation and delusion within the scene implying the characters self-deception. This is done in Stanley Kubrick's 'The Shining', creating discombobulation for both his audience and the main character.