180° Rule

The 180 degree rule is a technique which is used in order to make sure that the camera is always on the same side as the characters or objects in the scene.  To ensure the audience isn’t confused by a reverse in the characters position in the scene, the camera has to stay on one side of the horizontal line. If the camera crosses the line, the characters will appear to have swapped position. I've posted an image below to show how this works!