There are several 'rules' in video and film production that are regarded as 'Industry Conventions'
You may have heard of the phrase "don't cross the line". This is the 180 degree rule. It's actually a very straight forward convention and helps you to create flowing cut sequences when shooting with a single camera, but you want to create the impression that there are more cameras shooting the scene so that the viewer sees a cut sequence that flows naturally. This means that you will have to direct the actors or contributors to repeat the action over and over again while you shoot the action from different camera positions to get enough scene coverage.
This can be confusing when you are having to think of many aspects simultaneously, and is especially challenging when you are new to video production. However, the line is simply an imaginary line that you draw between two characters that you might be filming. The simple rule of thumb is that you choose one side of the line to film all your camera angles from if you want the cut sequence to flow naturally.
- Crossing the line - the 180 degree rule
So if we look at the diagram above you will see that the line in this instance is running vertically between the two subjects. Now, for simplicity's sake let's look at placing the camera anywhere within a semi circle on the right of the line, in the green portion of the image above.
The rule ensures that if you shoot from any position on that side of the line, then your sequence will cut seamlessly as if there were multiple cameras shooting the scene simultaneously.
However, if you were to jump to the other side of the line and edit that into the sequence, the shot would jar and would not flow naturally.
Here is a really useful short video about the rule by Peter John Ross.
[embed maxwidth="700" maxheight="394"]http://vimeo.com/2690589[/embed]