The 180 degree rule!

The 180 degree rule is a editing technique that is used so the viewer/audience are able to follow the action throughout the scene by establishing the character's time and space. The rule is specifically used in dialogue between two characters in a scene, and states that they should always have the same left/right relationship to each other. If this rule is not applied, it may somewhat confuse the viewers, and seem as if the characters have switched places.

By looking at this diagram, you can see that you can have as many cameras as you want on the green side, in this case the character in orange will always appear on the left-hand side and the character in blue will appear on the right-hand side. However, if the cameras were to switch sides, on to the red side, it would give the impression that the characters have consiquently switched sides; the orange character would appear to be on the right-hand side and the blue character would appear to be on the left-hand side.

When you can break the rule:

This rule can be flaunted through camera movement, by taking your audience through the 180 degree line. You can see where this rule is apparent in the clip below at 0:38 seconds through.

In this clip, you will see the camera move past the character, breaking the 180 degree rule. However, because you actually see the line being crossed, you are aware of the change and are not disoriented by the camera side swap.