180 degree rule

Crossing the line is a very important concept in video and film production. It refers to an imaginary line which cuts through the middle of the scene. Crossing the line changes the viewer's perspective and can cause confusion. For this reason, you shouldn't cross the line.

In this example the camera is located to the left. The imaginary line is shown in red. The resulting shot shows the subject walking from right to left, establishing the viewer's position and orientation relative to her.

In this example the camera has crossed the line. As you can see in the resulting shot, the view of the subject is reversed and she appears to be walking from right to left.