180° Rule

The 180° rule is a guideline stating that 2 subjects within the same scene should have a consistent left/right relationship to each other. If the camera were to cut through the imaginary line-known as the axis-connecting the two characters, this would be known as 'crossing the line'. Following on from that, the new shot taken from the opposite side is called a 'reverse angle'. If the 180° rule is broken, it can lead to the scene becoming very confusing for the viewer and consequently they may miss important parts of the scene.
The picture below shows the 180° rule: