Explanation of the 180 degree rule

This rule is usually applied when showing a conversation between two characters.

Once you establish the positions of two characters in a scene, for instance; if two characters are having a conversation at a table and say if the male is on the right and the female is on the left,  you have now established the 180 degree rule.

Here we can clearly see the woman is positioned on the right and the man is positioned on the left, so once the director cuts to an over-the-shoulder shot or a shot from the perspective of the character, and returns to the two-shot we expect to see the woman on the right and the man on the left of our screen.

As you can see from this over head shot above, the camera can only shoot from this side of the line, for the scene to adhere to the 180 degree rule.

Breaking the 180 degree rule

Now as you can see the director has placed two cameras either side of our imaginary line, violating the 180 degree rule as you will in the next few screenshots the effect of violating the 180 degree rule has on the consistency of storytelling.

Since the 180 degree rule has been broken both actors are facing the same direction breaking the continuity of the film, another downfall of violating the 180 degree rule is that the actors' eye lines  won't match, disorientating the audience.

Crossing the 180 degree line, not the 180 degree rule

For a director to cross the line and still remain in the perimeters if the 180 degree rule, the director must shoot the moment and use the clip in the edit where the camera crosses the 180 degree line to maintain continuity.