Crossing the Line (Reverse Cut)
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, from side to side with respect to the camera. Crossing the line changes the viewer's perspective in such as way that it causes disorientation and confusion. For this reason, crossing the line is something to be avoided.
In this example the camera is located to the subject's 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.
"Crossing the line" means shooting consecutive shots from opposite sides of the line.
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 left to right.
When cut immediately after the preceding shot, the effect is quite confusing.
Because of the sudden reversal of viewpoint and action, this is known as a reverse cut.
To prevent reverse cuts, set up the scene so you can shoot it all from one side. If you are using multiple cameras, position them on the same side.
In some cases crossing the line is unavoidable, or at least desirable enough to be worth the awkward transition. In this case you can minimize confusion by using a shot taken on the line itself to go between the shots, as illustrated below. This "buffer" shot guides the viewer to the new position so they know where they are. Although it's still not perfect, it's not such a severe jolt.
Sports & Multi-Camera Action
In live-action situations such as sports coverage, crossing the line is often necessary to obtain the best views. Sometimes this isn't a problem, especially if it's a view the audience is used to, but sometimes it can be very confusing (for example, a team suddenly seems to be playing in the wrong direction). This can be alleviated by either a graphic key saying something like "Reverse Angle", or a word from the commentator such as "Let's see that replay from a different angle".
The 180° Rule
The rule of line-crossing is sometimes called the 180° rule. This refers to keeping the camera position within a field of 180°.