From the Wikipedia: "Dice are small throwable objects with multiple resting positions, used for generating random numbers".

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Why do the opposing sides of a D20 generally add up to 21

Most D20s I own (with the exception of poorly made foam ones or MTG spin-down dice) seem to have all opposing sides add up to 21 (20 + 1, 15 + 6, etc). Why is this? Who started this?