180 Degree Rule
The diagram to the right shows how the
camera must be placed when filming a
scene between two characters.

 

 Line 'A' represents an imaginary line called the axis connects the characters. The camera must be kept on one side of this axis for every shot in the scene, this is so the first character will always be frame right of the second character, who is then always frame left of the first.

 

If the camera passes over the axis, it is called crossing the line or jumping the line.

 

This video shows what it looks like to the viewer when you break the 180 degree rule