In filmmaking we hear people talk about the 180 degree rule or refer to not crossing the line. So what does it mean to cross the line?

The red line pictured above represents the line of action. In this case  there are two actors talking to each other. As a rule all coverage of this should be shot from one side or the other of this imaginary line. Therefore the audience can form their mental map of where the actors are located in the scene and from the master they will know Jennifer is on the left and John is on the right. If we cross the line and go to a close up of Mary it will appear as if she has suddenly jumped to the other side. This can be quite distracting and can momentarily take the audience out of the story as they attempt to restablish where everyone is located. We saw this  our class excercise featuring the two ladies at the bar where Rob crossed the line and disturbed our spatial awareness of the scene. As a filmmaker you want to avoid anything that draws the audience out of the scene, this is why the 180 degree rule exists.

To elaborate, if we depict an actor walking down the street from left to right and then we cross the line and shoot them from the otherside they will be walking right to left.  It would then look like the actor had suddenly decided to walk back to where they came from. You could cross the line however if you displayed a shot of the actor walking around a corner.

It gets more difficult when you have a circle of actors sitting around a table. The ‘line’ will change as the dynamics of the converastion progresses. The director will have carefully plan the coverage so the audience can understand the layout of where everyone is sitting.