The 180 Degree Rule - Maria Beardall

The 180 Degree rule is a continuity editing technique that creates an imaginary line between characters and deciphers  the side that a camera may be allowed on. It creates a logical and easy to follow scene.

A rough definition is,
'It's called the 180 degree rule as you can't move the camera more than 180 degrees, the camera, therefore must remain the same side as the imaginary line. The rule should never be broken as it's for effect and if broken will confuse the audience.'

There is also a useful video on YouTube to help understand the rule:

Therefore we can take from this that if the rule is broken the scene will loose continuity and will seem odd and confusing to the viewer, yet if it is kept will look much more effective and due to this happening an 'Eyeline Match' will automatically follow - also another continuity editing technique.