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Every… well almost every… birth number is divisible by 11. It's an easy way to check if you've made a typo in your birth number. It's just a pity that we, as the Czech Republic, once issued several birth numbers that are not divisible by 11... There is an error in the video, of course, that a valid birth number must have four digits after the slash (except for the old birth numbers, which did not have a check digit and thus only had three digits after the slash).