The 180 degree rule

The 180 degree rule is a basic and fundamental rule in the film making process. This rule states that two characters or objects that are in the same scene must always have the same right and left relationship to each other. There is an imaginary line that the camera should never cross, demonstrated with a picture below. 

 Wikipedia

This rule is especially important when using shot reverse shots: if the 180 degree rule is broken in this context then the two characters will seem to be facing in the same direction. With this is mind we can create an example: If Bob (orange) is facing Alan (Blue) and the pink camera is shooting then Bob will be on the right of the frame and Alan will be on the left. This should be a continuous pattern and should never change- if the grey camera were to start filming in continuation of the pink camera then Bob will now be on the left, and Alan will be on the right. This will clearly cause confusion to the audience hence the importance of the 180 degree rule.