A TV image is made of hundreds of lines that are in turn made of hundreds of dots. When a colour television receives video, it decodes the signal into three separate signals, one for each of the primary colours. Red, green, and blue. The colours you see on your television are created by adding various levels of these three colours. Inside the TV tube are three electron guns, one for each colour that emit beams which scan the screen in a series of lines. In each line are rows of dots. The dots are in groups of three red, green and blue, which are made of colour phosphorus that glow when struck by the electron beams. As the beam scans line by line across the tube, triggering the tiny dots, a colour image is reconstructed. To make sure that the beams from the electron guns hit the right coloured dots, a black shadow mask with holes for each dot is placed between the dots and the guns. And in American TV, the beam scanned 525 lines 30 times a second. The combination of the speed of the scan and the lag effect of the glowing phosphorus creates the illusion of a continuous image on the screen.