180 Degree Rule

Another rule that is important for filmed dialogues. It states that the left/right relationship of to talking people shouldn't change during a scene, which means:
When in one shot Person 1 is sitting on the right and Person 2 on the left, they should still sit like this in the following shot.
It's called the 180 degree rule, because an imagined line that runs through both people shouldn't be crossed by the camera, which should only be placed in the 180 degrees on one side of that line. 

For example: 

If two people are shown talking like this,

they shouldn't be shown like this

in another shot.

So this is the rule:

The camera should only be placed in the 180° on the green side of the line.