180 Degree Rule

The 180 degree rule is one of the most basic rules that film and television makers use when it comes to how they set their shots out. If you picture a circle and half it you get a semi-circle, visualise the line (see diagram below) this line is your boundary.  You place your cameras in one side of the circle and you stay there. You cannot pass that line or you'll ruin the illusion of reality and confuse the audience.

the YouTube link below clearly demonstrates why you cannot cross the line giving you good examples with film references also.