180 degree rule

This is when directors use an imaginary line: "the line of interaction" to film an interaction normally between two people. They keep the camera on one side of the straight line when they turn to film the different characters, this is to ensure that it looks like the characters are conversing with each other to the audience, this may help to make the audience feel as though what they are seeing is real and therefore become more involved or affected by the storyline.
Demonstrated in the picture the camera always stays on the right side to keep the audiences view point, maintaining continuity.