180 Degree Rule

The 180 degree rule is used very often in films and other media related things.
What is it?
The 180 degree rule is where the cameras are placed to show the correct shots of characters so that the characters eyelines match up. In the diagram below it shows two characters and it shows where the correct angle for each camera needs to be to create the 180 degree rule, the camera with the cross through it shows that the camera should not be there so therefore it is in the wrong place.