180 rule = The Action Line = The Line
The Classic Style of shooting (or The Hollywood Style) says that when there are two characters in a scene there is an imaginary line that joins the point between their eyes and the camera once established on one side of that imaginary line it cannot cross it. On set everyone refers to it as “crossing the line”.
Let’s imagine what happens if the camera crosses the line.
In the first image you see the two characters facing each other: the guitarist looks to Camera Right, the saxophonist looks to Camera Left.
Next figure, the camera shows a Close Up of the guitarist – he still looks Camera Right. Next figure the saxophonist looks Camera Left – the same as in the Wide Shot.
When the camera crosses that imaginary line drawn in the first figure, the saxophonist’s eyeline will change, now looking Camera Right. If this shot is cut together with the one of the guitarist it will seem the two do not make eye contact.
A perception issue, really.
Same rule applies to Screen Direction: when a character walks Left to Right in one shot, that character needs to continue walking the same screen direction, Left to Right in the following shot. Otherwise it’s confusing to watch, inconsistent.
The reason the camera needs to follow these rules is to make the experience of watching a movie similar to the reality. Try to imagine you are watching a conversation from afar. You don’t change your position, always watch by looking from one talking person to the other. It makes sense that the same rule applies for the camera, but cheating the angles for tighter coverage to see the characters’ faces a bit better.
Things become a bit more complicated when there are a few more characters on screen. Keep in mind that most times the actors move and the eyelines can change a few times during the same shot. It always helps to draw diagrams and mark the dotted lines between characters.
By the way, this line cross business is your job! Always talk to the DOP about it during coverage and make sure the camera doesn’t cross it, unless it’s intentional.