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To be specific, I am just curious as to why does our number system have 10 different digits?Just for an example, why did it not end at 7? (it is the greatest prime number before 10, there are 7 days of week). Or similarly any other digit
If we assume the digits were derived from the number of fingers(including thumbs) on our hand, then why is it so that 0 represents nothing, 1 represents one finger, then for the 10th finger, we don't have a unique digit, but have to ADD one more to nine.

EDIT: My question also aims to clear one more thing. If it is derived from the number of fingers, then why don't we have a special symbol for the 10th finger,

explorer
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    When did $7$ days come in to being? but I think people had $10$ fingers and thumbs. Just a theory. I will google ;). – Chinny84 Jul 29 '15 at 16:16
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    I once helped prepare http://www.gutenberg.org/ebooks/16449 – Hagen von Eitzen Jul 29 '15 at 16:16
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    @Chinny84: There is nothing specific with 7. But what is the real significance of number 9. I think I should edit my question. – explorer Jul 29 '15 at 16:19
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    It's somewhat unclear what you're asking. Base 10 is arbitrary; different cultures used different bases. The symbols "0" and "1" are also arbitrary symbols (they don't represent fingers) which are historically derived from the Hindu–Arabic numeral system. We don't have a unique digit for ten because otherwise we would be using base 11. – Insert Pseudonym Jul 29 '15 at 16:19
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    @InsertPseudonym That book Hagen von Eitzen prepared shows pretty clearly that although different cultures use different bases, the bases are always related to 10; typically if they are not base 10 they are base 20, or an elaboration on base 5, or an elaboration on base 10 like the Babylonians' 60. They are never, for example, base 12 or 8. – MJD Jul 29 '15 at 16:30
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    It's no coincidence that the Latin word for finger is digitus. – MPW Jul 29 '15 at 16:44
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    Mainly because ordinary people have ten fingers. If you also count your , toes, you have a basis 20 numeration system, like the Celts. That's why in French, one says quatre-vingts for eighty. – Bernard Jul 29 '15 at 16:50
  • Because "Dec" means 10. Like in decade. decibel (base 10)... – Alec Teal Jul 29 '15 at 17:04
  • @AlecTeal: Realising my mistake, I have edited the question. – explorer Jul 29 '15 at 17:07
  • Because otherwise it'd be base 11? As the top answer says 10 is one lot of all my fingers. – Alec Teal Jul 29 '15 at 17:08
  • @AlecTeal: I edited the question because it was marked duplicate. I was already clear with the answer. Thanks! – explorer Jul 29 '15 at 17:09

2 Answers2

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If I show you both hands two times and then three fingers, I show you a total of $23$ fingers, where the $2$ and the $3$ excellently represent the two double-hands and the three fingers. Likewise, one double-hand and no extra fingers should therefore be represented as $10$, not with yet another digit.

Hagen von Eitzen
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  • Thanks! I really feel dumb for not thinking this way at all! – explorer Jul 29 '15 at 16:32
  • In fact, "everybody" did it this way. Just about every system of numeration I can think of makes a special symbol for "one handful of fingers" and/or "two handfuls of fingers". As just one example, the "I", "II', "III" (and at one time, "IIII") in Roman numeration represent fingers, up to "one hand's worth", "V" (a stylized hand omitted the three central fingers), then back to adding fingers until reaching "two hands' worth", "X" (two stylized hands placed "heel-to-heel"). If you look up the practices of other folk going back millenia, they pretty much all "turn over" at ten. – colormegone Jul 30 '15 at 01:35
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Prime numbers are not really convenient as bases for number systems. There is rather an advantage in having divisors, which is likely why some other number systems where based around $60$ and $12$ is a also common (recall a dozen, and the timescale).

The reason for base $10$ being the ten fingers is plausible, the fact that it is a kind of convenient size and has some divisors might have contributed to it.

The reason why ten fingers lead to base ten rather than to base eleven is nicely explained in the other answer.

quid
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