The 180 degree rule

The 180 degree rule is a cinematography guideline. The basic guideline regarding on-screen spatial relationship between two characters or objects. The guideline states that two characters in a scene should maintain the same left or right relationship to each other. There is a invisible line, if the camera crosses it, it is called 'crossing the line'. The invisible line stops the camera from crossing the axis, if it does, it is no longer the 180 degree rule. This is known as crossing the line or jumping the line.

The image shows the axis between the two characters. The 180 degree green arc is where the camera can be placed. if the camera moves over to the red arc, it is no longer the 180 degree rule. If cutting from the green arc to the red arc the characters need to switch places on the screen from left to right or right to left, otherwise it is no longer the 180 degree rule. 

If the line is crossed it becomes very similar to shot reverse shot, which is another camera technique.