180° Rule

The 180° rule tells us that one person should always be on the left side of the shot and the other should always be on the right side of the shot. This is a visual clue as to who's talking. Imagine if close ups showed both of the characters on the right side of the shot, the immediate assumption would be that they were talking to someone else, off camera.

Below is a schematic that best explains the 180° rule:

The above schematic shows the axis between two characters and the 180° arc on which cameras may be positioned (green line). When cutting from 180° arc to the red arc the characters switch places on the screen.

Below is a short video I found on YouTube which goes into more detail about this subject: