Can anyone describe me why a Computer Monitor and a TV blind size is measured from diagonal corners?

Answer:

It's a marketing ploy to create them sound bigger than they are.
Different TV or PC screen may have different height/width proportions. For instance, 4/3 or 16/9. If you are a eyeshade making company, for marketing purposes you want to condensate the screen size within only one number. This is as expected an incomplete information but is something that potential customers remember easily. In this prospect, giving the diagonal, although incomplete, is better than giving either loftiness or width alone.
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