180 degree rule

Is a rule that the camera should be placed somewhere inside 180 degrees on a particular side of the invisible line of a shot that* has two people filmed in sequence. If the camera crosses the line, it will confuse the audience, because it makes it look like the people are switching places as one watches the film.



Here's a clip from "reggie perrin" from 07:23-08:36, it has perfect example of 180 degree. As you can see the eyeline matches, they are both between the line and the camera stays within the semi circle.