The 180 Degree Rule.

Recently I was introduced to the importance of the 180 degree rule when filming, and learnt the concept of it. It basically means that when filming from two different sides, E.G. a shot-reverse-shot of a conversation between two people, it is imperative that you film from the right side of each person or the left side of each person. If this rule isn't followed then it will appear that the two characters have switched places, therefore conflicting with continuity.