A technique used in filmmaking regarding two characters in a scene. It is a part of continuity editing. It is used as to not confuse the audience. The rule is sometimes deliberately broken by the Director to create an effect – usually to create some sort of disorientation. What is the 180 Degree Rule
The camera cannot “jump over” an imaginary line drawn between two characters or objects. The camera can go anywhere on one side of the line, but not the other. The line can only be crossed for effect. If the line must be crossed there needs to be a shot in between to avoid confusing the audience. The Rules
The camera may only be placed anywhere on the green side of the imaginary line (axis). The man in orange will always be on the left side of the frame if the line isn’t crossed.
It is clear to see here that when the line isn't crossed that the red piece will always be on the left side of the frame. Images 1, 2 and 3 are all on the same side of the line, and couldn’t be used in sequence with image 4 unless the Director is using it for effect.
In Peter Jackson’s “The Lord of the Rings” the character Gollum has a conversation with himself - his “evil side” and his “good side”. It is used to emphasise Gollum’s duel personality. Filmic Example
The 180 degree rule is also broken in Stanley Kubrick’s film “The Shining” in order to create disorientation within the audience.