180° Rule

The 180 degree rule, is a general consensus in film creation that allows and ensures that two characters within a scene will always have the same left/right relationship with each other.  

A quote from online

 "If the rule itself was broken, then the two subjects within the cameras focus would be shown from opposite sides and this would mean it would be in a reverse angle which doesn’t have the same effect which is why the rule exists" 

Following this rule isn't always the best option though, it can confuse the audience which is not what the director wants. A diagram is shown below explaining the 180 degree rule.

This example from ‘The Shining’ shows a bathroom scene shows the camera itself breaking the 180 degree rule. As said before, it creates an interesting effect; the view of the scene is now completely different and more interesting. Even though the editing is much more obvious now but with continuity editing, it makes it invisible.