Continuity in Film: 180 Degree Rule

The 180-degree rule states that two characters in a scene should maintain the same left/right relationship to one another. The rule states that the camera should remain on the same side of the invisible axis. This is because the axis connects the characters; by keeping the camera on one side of the axis for every shot in the scene, the first character is always on the right of the second character and vice versa. This helps to not disorient the audience.

For example, in a dialogue between two characters, if the camera remains on one side of the axis, the relationship between the two characters will continue from shot to shot, even if one of the characters is not on screen. Moving the shot to the other side of the characters will reverse the order of the characters from left to right will confuse the audience into thinking the characters are talking to themselves instead of to each other. Another example is in a car chase. If a vehicle leaves from the right side of the frame during the first shot, it should then re- enter from the left side of the frame in the next shot. Leaving from the right and entering from the right will also create a sense of confusion within the audience. 

When the camera passes over the invisible axis, it is called crossing the line and the shot becomes a reverse angle. A reverse angle will disorient the viewer by showing an opposing viewpoint of the scene, by altering the perspective of the scene and the relationship between the characters established in the original shot. To avoid this disorientation of the audience, the camera could move in the scene to show the audience that it is from a different angle or viewpoint.