The 180° Rule

In filmmaking, the 180° rule is a guideline that means the action should be all shot from one side. The diagram below helps explain this:

 As shown by the diagram, the actors are connected by a line (the line of action). The rule means that all cameras must stay on one side of this line (green cameras are in the right place, whereas the red one is wrongly placed). If a shot from a green camera cut to a shot from the red camera, the rule would be broken and the scene would look unnatrual because the characters would swap places on screen.

We made a short video breaking the rule to learn about it and to demonstrate what it looks like when it's broken: