The 180 degree rule, is a guideline for film makers, which assist them in creating a simple, easy to follow film which viewers can understand without trouble. It creates a relationship with the entities in the scene, by placing them in a certain order of positions on the screen.
The guideline utilizes a “axis”, which is a straight line between the two (or more) entities. The idea is that the camera should never move over this line, and should stay in the 180 degree arc for the entire scene. The rule usually comes into practice when shooting a “two-shot” scene, however it can be utilized in other types of shot, or practices, such as “shot-reverse-shot”, during dialog.
Some experienced filmmakers use a technique called “shooting in the round”, which uses all 360 degrees in the scene, however this difficult to perfect, and can easily distract the viewer and cause confusion later in the movie. When filmmakers “jump the line”, entities in the scene swap places. This could cause a character to seem to walk the wrong direction, which could cause confusion. For example, in a scene where a paramedic is scripted to quickly run to an important incident, if the running scene “jumps the line”, it may display the paramedic as running away, which would obviously cause confusion to the viewer.
Scenes which follow the guideline usually flow a lot smoother, and are easier for the viewer to become submerged in. These scenes also are received as more interesting, as they can provide varied shots, without breaking the continuity.
Although the guideline is usually used to orientate viewers, it can also be used against them. This practice is called a “reverse cut”, and although not used to confuse the viewer, it is used to disorientate them. It does this by presenting an opposite viewpoint of the scene’s action. An example of where this may be used, is in a street fight. The fight may be displayed on one side, however a “reverse cut” may be used to show a bystander’s view from the other side of the street.