180 degree rule

The 180 degree rule is a basic guideline for film editing. It states that two characters in the same scene should always have the same left & right relationships with each other. This means that if the camera passes over an imaginary axis connecting the two subjects, it is called crossing the line.
Below is the basic diagram for the idea. It shows the axis between the two characters and the 180 degree arc in which the cameras can be positioned.