The master producers follow an established set of rules for a reason - to move the story along in a logical way that won't confuse the viewer. There are many things that we "know" about movies simply from having seen lots of them. We "know" for example that when the camera pans across a vast expanse of city and zooms in on a sign on a building that says International Widget, that the action that takes place after the cut is happening at International Widget. There are a number of rules we all know subconsciously even if we've never been told them. The 180 degree rule is one of them. The rule states simply that when filming (or videotaping) two elements (people, cars, armies, sports), the camera should stay within a 180 degree field and not "cross the line".

Establishing a Map

Viewers map out the logistics of a scene in their heads, we take subtle clues from the screen and use them to build a visual representation of the spacial relation of the objects we're seeing.

Close your eyes and see if you can imagine the layout of the Brady Bunch house or another building from fictional television, do you know which rooms are to the left or right of others? Chances are you do, even if the actual sets aren't anywhere near one another, because the movie makers have spent some time trying to make you comfortable and let your mind interpret a reasonable geography. The 180 degree rule is one of those things that keeps viewers comfortable by establishing where people are in a scene, including the viewer.

Examples

Let's imagine a very simple scene two people sitting across from one another at a coffee shop, having a conversation. The 180 degree rule tells us that one person should always be facing camera left and the other should always be facing camera right. This is a visual clue as to who's talking. Imagine if close ups showed both of the characters looking to camera right the immediate visual assumption would be that they were talking to someone else, off camera.

Now think of something where direction is very critical a football game. The Sharks are trying to get the ball to the right of the screen, the Jets are trying to get the ball to the left of the screen. When you see someone running in one direction or the other, you automatically know which team they're on regardless of whether you can see their face or remember what colors they're wearing.

The 180-degree rule is important even in scenes that have a single person - imagine a character saying "I'm going to the store" - we cut to a shot of her walking right to left and then cut to a shot of her walking left to right - the implication is that she's headed back, she's either been to the store, or forgotten her car keys. Simply deciding where to put the camera has given us a shortcut - we don't need a store, cutting back and forth tells the viewer that they've been there.