In continuity editing, the 180° line (a.k.a. axis of action) is an imaginary line passing through the two main actors (or actor and object) of a scene, defining the spatial relationship among scene elements as being to the right or left of this imaginary line. The 180° rule suggests that the camera should not cross the imaginary line at a cut, as this would disorient the viewer by reversing the spatial relationships.
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This rule is among the most important rules of continuity because we expect the relationships of the things presented to us to remain constant. If the camera breaks the line of action (that is, the imaginary line that restricts the camera) and cuts to the other side of the line, it will appear as though the characters have switched places. In most situations, this is considered bad form, although it is admissible if the cut is so blatant that we can understand that the position of the characters has remained constant. The camera may move during the shot from one side to the other in order to re-establish a new imaginary line. See continuity editing.