There are 4-5 different names for the 180˚ Rule depending on when it was taught. The rule was first acknowledged in the early 1920s. The film attributed with its origin is the 1925 film The Battleship Potemkin.
The below diagram illustrates the rule. The dotted line is an imaginary line that represents the action line or the camera line. The camera is aloud to be set up anywhere on one side of the line, giving it 180˚ of understandable angles that can be shot. The red camera broke the rule by crossing the line, which will create an image that may not be readily understood and can confuse the audience.
By viewing the shots each camera position captures will help bring visual clarity to the rule. The below picture is the establishing shot captured by the “2 Shot Both” camera. The image helps the audience to understand that the officer in the purple outfit is on screen left, looking right. The audience also understands that when the purple officer is looking screen right, he is looking at the red officer. And, if he is looking screen left, he is looking away from the red officer.
The next picture is the “CU Purple” camera that captures the close-up of the purple officer. Because he is looking screen right, the audience knows he’s still looking at the red officer even though he is not on screen. It is a simple illusion that our mind fills in to create continuity of story and understanding.
The next picture is the “CU Red” camera that captures the close-up of the red officer. Because he is looking screen left, the audience knows he’s still looking at the purple officer even though he is not on screen. By cutting back and forth between the two close-ups, the audience has the illusion that they are talking face to face.
The next picture is the “Red” camera that captures a close-up of the purple officer from across the action line. While the purple officer is still looking at the red officer, the audience thinks he turned around and is looking away from the red officer because of the direction he is facing, which is not the same as the establishing two shot.
This flipping of the image is unsettling to the audience and creates significant confusion. This pulls the audience out of the story until they can reorient themselves to the virtual surroundings they are witnessing.
Now that you understand the rule and why you can’t break the action line, there is a way to cheat the camera placement so you can move all away around the full 360˚ circle. This can be accomplished by rotating the action line.
Rotating the axis is a difficult skill that requires a few extra rules for understanding. First, whenever you change shots, the next camera has to be placed at a minimum angle of 30˚ difference to the previous angle. If this is not done, then the shot will appear to be a jump cut, which can be disorienting to the audience.
The second rule is that you can’t move from the “CU Red” camera to the “Red” camera because it would be visually jolting for the audience. You can, however, make the movements incremental between 30˚ and 90˚ at a time. This means that you can move from the “CU Purple” camera to the “Red” camera without jolting the audience. But, once you’ve arrived at the “Red” camera, you can’t go back to the “CU Red” without moving back incrementally as you rotate the axis back.
In other words, once you’ve moved from the “CU Purple” camera to the “Red” camera, the action line is now perpendicular to its starting point. This gives you a new 180˚ rule that allows you to use any of the camera setups from the right side of the diagram instead of the cameras across the bottom of the diagram.