The 180 degree rule

One simple principle when shooting subjects is considering ‘the line’, known as the 180-degree rule. For some filmmakers and/ or videographers this may come naturally, but this is one major principle, no matter your degree of experience or knowledge, that every camera operator needs to always consider; otherwise his or her work will appear amateurish.
The 180-degree rule is an imaginary line or the axis of action drawn across the frame or screen. It is the location where not only action or the direction a subject is moving, but it is also the sightlines, or the character’s direction of vision. Once shooting has begun, the camera operator needs to stay on one side of the line in which they initially started shooting that particular action.

To break the 180 degree rule you move the camera, while shooting, across the line. For example, a Steadicam operator tracking around the subject(s) while shooting, thus creating a continuous flow of action--does not break the 180-degree rule. The operator may stop anywhere, but once stopped, there’s always a line.
The main thing you must consider, whether planning your shots while storyboarding or while blocking the set, is to consider the 180-degree rule. It’s simple to follow! Once the line is followed, you automatically create a continuous flow of action and a logical sense of direction within your video.