The 180 Degree rule

The 180 degree rule
Imagine drawing a line following the eye sight of two people in a conversation facing each other.
then if you choose to film on one side of that line you have to stick with that side for the whole dialogue, unless there is an interruption with another character. if you go against the 180 degree rule the editing is very clear and makes the audience clearly see that this film has been edited and loose interest in the dialogue. also if you follow the rule the transition's between each shot of each character runs more smoothly and is more appealing to the audience.

This diagram explains the rule in a more simple way.