180 Degree Rule

The 180 Degree Rule is where the camera remains on one side of an imaginary line that divides the characters. If the camera crosses the line it can become confusing for the audience as it makes it look as though the characters are swapping places. If the camera does cross the imaginary line it will be for a specific purpose. 

This image gives a good representation of the 180 degree rule.  

The basic idea is that the camera should always film on one side of the line of vision so that the two characters are always on the same side of the frame, making the scene continuous. 

It is often used in car chase scenes. If the car leaves on the right side of the frame, in the next shot it should be on the left side. This all adds to the continuity of the scene.