180 degree rule

The 180 degree rule is a filming guideline that says that characters in a scene should have the same left right relationship with each other. When the two characters are facing each other an imaginary line must be draw, the line usually runs straight down the middle of these two characters as shown in the image below.

Once this imaginary has been set and the filming has begun the camera must stay on the same side of the line. If the camera is switching between faces and suddenly crosses to the other side of the line, the characters will appear as though they are facing the same direction and it will not look as though they are talking to each other.