CRT monitors use the tube to get the image on the screen, and flat panel displays use liquid crystal diode (LCD). CRT shoot electrons through a grid on the screen, whereas LCD monitors are made up of diodes, which are simply flashes at different intensities. That is why the CRT monitors occupy much more space than LCD screens.

So why CRT is less than the LCD screen, which, presumably, the same size? It is because CRTs do not use the entire tube, which prevents the edges of the screen is stretched and becomes blurred. Therefore a 20 "CRT will be viewable area of less than 19 inches. Flat panel displays, on the other hand, do not use the phone, so it should be visible region of exactly 20. This means that 19-inch LCD monitor, as a rule, are the visible region, which is more than 20 "CRT display. Here is a precise marketing.