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 within a scene. By keeping the camera on one side of an imaginary axis between two characters, the first character is always frame right of the second character. Moving the camera over the axis is called jumping the line or crossing the line; breaking the 180-degree rule by shooting on all sides is known as shooting in the round.
The 180-degree rule enables the audience to visually connect with unseen movement happening around and behind the immediate subject and is particularly important in the narration of battle scenes.
In a dialogue scene between two characters, a straight line can be imagined running between the two characters, and extending to infinity. If the camera remains on one side of this line, the spatial relationship between the two characters will be consistent from shot to shot, even if one of the characters is not on screen. 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. For example, if a character is walking in a leftward direction and is to be picked up by another camera, the character must exit the first shot on frame left and enter the next shot frame right.
A jump cut can be used to denote 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.
The imaginary line allows viewers to orient themselves with the position and direction of action in a scene. 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.
There are a variety of ways to avoid confusion related to crossing the line due to particular situations caused by actions or situations in a scene that would necessitate breaking the 180-degree line. 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.
Another way to allow for crossing the line is to have several shots with the camera arching from one side of the line to the other during the scene. That shot can be used to orient the audience to the fact that they are looking at the scene from another angle. In the case of movement, if a character is seen walking into frame from behind on the left side walking towards a building corner on the right, as they walk around the corner of the building, the camera can catch them coming towards the camera on the other side of the building entering the frame from the left side and then walk straight at the camera and then exit the left side of the frame.
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. Carl Theodor Dreyer did this in The Passion of Joan of Arc; Stanley Kubrick also did this, for example, in the bathroom scene in The Shining. The Wachowskis and directors Jacques Demy, Tinto Brass, Yasujirō Ozu, Wong Kar-wai, and Jacques Tati sometimes ignored this rule also, as has Lars von Trier in Antichrist. In the seminal film of the French New Wave, À bout de souffle ("Breathless"), Jean-Luc Godard breaks the rule in the first five minutes in a car scene that jumps between the front and back seats, improvising an "aesthetic rebellion" for which the New Wave would become known. When the rule is broken accidentally, or for a technical reason (such as the inability to place a camera physically in the correct position), there are ways for the editor to attempt to hide the mistake. The editor may pre-lap one or two words of dialog before the cut, so that the viewer is concentrating on what is being said and may be less likely to notice the rule-breaking cut.
Some styles used with the 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.
In the Japanese animated picture Paprika, two of the main characters discuss crossing the line and demonstrate the disorienting effect of actually performing the action.