This schematic shows the axis between two characters and the 180o arc on which cameras may be positioned (green). When cutting from the green arc to the red arc, the characters switch places on the screen.
In film making, the 180-degree rule is a basic guideline regarding the on-screen spatial relationship between a character and another character or object with a screen. An imaginary line called the axis connects the characters, and by keeping the camera on one side of this axis for every shot in the scene, the first character is always frame right of the second character, who is then always frame left of the first. The camera passing over the axis is called jumping the line or crossing the line.
The 180-degree rule enables the audience to visually connect with unseen movement happening around and behind the immediate subject and is important in the narration of battle scenes.
You can clearly understand this 180-degree rule by watching this video:
If a shot following an earlier shot in a sequence is located on the opposite side of the 180-degree line, then it is called a reverse cut. Reverse cuts disorient the viewer by presenting an opposing viewpoint of the action in a scene and consequently altering the perspective of the action and the spatial orientation established in the original shot. The movement in the scene can be altered, or cameras set up on one side of the scene so that all the shots reflect the view from that side of the 180-degree line.
To minimize the jolt between shots in a sequence on either sides of the 180-degree line, a buffer shot can be included along the 180-degree line separating each side. This lets the viewer visually comprehend the change in viewpoint expressed in the sequence.
In professional productions, the applied 180-degree rule is an essential element for a style of film editing called continuity editing. The rule is not always obeyed. Sometimes a filmmaker purposely breaks the line of action to create disorientation.
Some styles used with 180-degree rule can elicit an emotion or create a visual rhythm. By moving the camera closer to the axis for a close-up shot, it can intensify a scene when paired with a long shot. When the camera is moved further away from the axis for a long shot after a close-up shot, it may provide a break in the action of the scene.
For example, Dreyer did this in The Passion of Joan of Arc; Stanley Kubrick also did this in the bathroom scene in The Shining. In the Japanese animated picture Paprika two of the main characters discuss crossing the line and demonstrate the disorienting effect or actually performing the action.
Videomaker describes the breaking of this 180 Degree Rule in this video:
Examples of 180 Degree Rule
In a dialogue scene between two characters, a straight line can be imagined running between the two characters, and extending to infinity. Shifting to the other side of the characters on a cut will reverse the order of the characters from left to right and may disorient the audience.
The rule also applies to the movement of a character as the “line” created by the path of the character. A jump cut can be used to detonate time. If a character leaves the frame on the left side and enters the frame on the left in a different location, it can give the illusion of an extended amount of time passing.
Another example could be a car chase; if a vehicle leaves the right side of the frame in one shot, it should enter from the left side of the frame in the next shot. Leaving from the right and entering from the right creates a similar sense of disorientation as in the dialogue example.
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