Definition of the 180 degree rule: The rule that says a camera should be put somewhere inside 180 degrees on a particular side of the invisible line of a shot containing two people filmed in sequence. If the camera crosses the line, confusion results for the audience as, to them, it makes it look like the people are switching places as one watches the film.
Example of the 180 degree rule:
Here the 180 degree rule is used to give the impression of two different people speaking to each other to the audience so they aren't confused by gollum seemingly talking to himself.
Example of breaking the 180 degree rule:
They deliberately break the 180 degree rule here to make this scene more confusing and disorienting to the audience.
