A TV works by sweeping its electron beams from one side of the screen to the other, then repeating this just below the line it has just drawn, and so on from the top to the bottom of the screen. With PAL systems, this creates 576 visible lines (480 for NTSC). These are called scan lines. If your screen is big and you are sitting too close to it, you can sometimes actually see the scan lines. This is rarely a problem on standard CRT TV sets, but often a problem with rear projection TVs. The solution is to sit further away or get a smaller, or better designed, viewing device.