The size of a mobile phone display or of a computer monitor is given by a single length in millimeters, centimeters or inches. This measurement is taken diagonally, so it gives the largest straight-line measurement that can be obtained from the display. The quoted screen size, being a diagonal, is larger than the height or the width of the display.
Perhaps surprisingly, two displays with the same diagonal may have different sizes because they differ in “aspect ratio” (the ratio between the vertical and horizontal size).
Older monitors usually have an aspect ratio of 4:3 (pronounced four to three). If such a display has a diagonal of 15 inches, it will have a height of 9 inches and a width of 12 inches (for a total area of 108 square inches). A monitor made for HDTV (high definition television) would have an aspect ratio of 16:9. If such a display has a diagonal of 15 inches, it will have a height of 7.35 inches and a width of 13.07 inches (for a total area of 96 square inches). A widescreen display will always have a smaller area compared to a standard screen of the same diagonal.
There’s an additional complication for CRT (cathode ray tube) monitors. In this case, the custom is to measure the diagonal of the front glass of the display tube. This can be misleading because the image does not go to the edge of the glass, which is covered by the bezel of the monitor case.
Therefore, a “21 inch” CRT will have a smaller image than a “21 inch” flat panel LCD monitor. It’s not possible to say how much smaller it will be, because different ways of making cathode ray tubes result in different amounts of “dead space” around the edges. Sometimes the specifications will include a separate measurement for the “visible” diagonal, and this is the one that is meaningful to the end-user.
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