|The 180 Degree Rule. Wikimedia|
"Crossing the Line" is also known as the 180-degree rule, and it is one of the fundamental rules of cinematography.
Like most rules of film-making, the only way to really understand the 180 Degree Rule is to break it, figure out what went wrong - and then work out how to fix it next time.
Let's say you have a shot with two characters talking to one another. Draw an imaginary line between the two characters. This is the "line" that we should not cross.
Blue on the left, Orange on the Right
On one side of this line we place the camera (see image above). In the example, the camera is on the left hand side, pointing at the two characters. We will see Mr Blue on screen left, and Mr Orange on screen right. As long as we keep the camera on the same side of the line, it doesn't matter where we put it, Blue will be on the left, Orange will be on the right.
By keeping the camera on one side of an imaginary axis between two characters, the first character is always frame right of the second character.
Moving the camera over the axis is called jumping the line or crossing the line.
Crossing the Line Confuses the Viewer
The problem with crossing the line is that it disorientates the viewer, and confuses them. Since your goal as an animator (and also as a director and cinematographer) is not to confuse your audience, you want to avoid crossing the line. The simplest way to do this is always to keep the characters on one side of the screen or the other - never let them switch places.
Maintaining Screen Direction
Crossing the line is almost (but not exactly) the same thing as maintaining screen direction, which is a very similar and related concept; the two are usually considered together. If you maintain screen direction (eg, Orange on the right, Blue on the left) correctly, then you should also avoid crossing the line as well.
Crossing The Line in "Gladiator"
To see how Ridley Scott crosses the line in the classic film "Gladiator" (but does it with style and skill), watch the short clip below.