Unless the narrative calls for it, audiences don't usually like being disoriented while watching films. This is why the 180-degree rule exists, to establish spacial relationships on-screen and provide what is essentially a coherent map of where all characters and objects are within the frame. If you're not familiar with this important filmmaking concept, Jordy Vandeput of Cinecom is here to not only break it down for you, but to also explain how the rule can (and sometimes must) be broken.
The 180-degree rule is a guideline that gives your audience information about on-screen spacial relationships between characters and objects. Picture an overhead schematic of your scene and then draw a line right down the middle of it. This line is called the "axis" and it divides your frame into two sides. The 180-degree rule says that you must choose one of these sides to shoot from and must never cross the axis and shoot from the other.
Of course, that's a very simplified explanation of the rule. There are many instances in which you would cross the line to shoot from the other side—a cinematic move that actually requires several shots, usually ones from right down the line, in order to do it correctly without disorienting your audience. Note, however, that doing this isn't breaking the 180-degree rule, but there are instances in which you would intentionally break the rule altogether, like to communicate chaos, confusion, or haste.
When you're on set, it's easy to get confused about which side you're supposed to be shooting on. I highly recommend drawing up shot lists, storyboards, and/or overhead schematics that show you exactly where you've placed the axis.
What are some reasons filmmakers would want to break the 180-degree rule? What's your favorite 180-degree rule break in a film? Let us know down in the comments.