Computer Monitors' Scree size
The size of an approximately rectangular display is usually given as the distance between two opposite screen corners, that is, the diagonal of the rectangle. One problem with this method is that it does not take into account the display aspect ratio, so that for example a 16:9 21 in (53 cm) widescreen display is far less high, and has less area, than a 21 in (53 cm) 4:3 screen. The 4:3 screen has dimensions of 16.8 × 12.6 in (43 × 32 cm) and area 211 sq in (1,360 cm2), while the widescreen is 18.3 × 10.3 in (46 × 26 cm), 188 sq in (1,210 cm2). For many purposes the height of the display is the main parameter; a 16:9 display needs a diagonal 22% larger than a 4:3 display for the same height.
This method of measurement is inherited from the method used for the first generation of CRT television, when picture tubes with circular faces were in common use. Being circular, only their diameter was needed to describe their size. Since these circular tubes were used to display rectangular images, the diagonal measurement of the rectangle was equivalent to the diameter of the tube's face. This method continued even when cathode ray tubes were manufactured as rounded rectangles; it had the advantage of being a single number specifying the size, and was not confusing when the aspect ratio was universally 4:3.
A problematic practice was the use of the size of a monitor's imaging element, rather than the size of its viewable image, when describing its size in publicity and advertising materials. On CRT displays a substantial portion of the CRT's screen is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's "safe area" due to overscan. These practices were seen as deceptive, and widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable area.