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In: 4237
It’s inherited from old english.
Probably because 12 was an important number back in the days, with dozen and a gross (144, 12×12) being important trade volumes.
This is because it was easier to do math with 12 since it can be divided evenly by 2, 3, 4 and 6 rather than just 2 and 5.
Those terms come from the Old English words endleofan and twelf. This comes from an earlier construction of ainlif and twalif where they are referring to a remainder, like saying “ten and one” or “ten and two”.
Why stop at just eleven and twelve? This is probably due to counting up to a dozen being all that the typical person would be required to do, and so terms used commonly would stop there. Contributing to this may be that a way of counting on one’s fingers was to use the thumb to point at each joint of the fingers of one hand. Each of the four fingers has three joints, adding up to twelve.
Twelve also has more factors than ten which could explain it being commonly used. Ten has only 1, 2, 5, and 10 as factors, while twelve has 1, 2, 3, 4, 6, and 12. If you want to easily divide something evenly then starting from twelve is more convenient than ten.
This is more of a linguistic question rather then a maths question. From a linguistic point of view the decimal system is a fairly modern system. It was mostly used in academic circles until maths were introduced to lower levels of education. Most people used fractional maths in their daily life, and the decimal system is horrible for fractions. Instead people counted in dozens, i.e. 12. You can still find this everywhere with lots of packages in the supermarket being packets of 6,12 or 18. Regular people would therefore use the numbers 1-12 very often. You would not buy thirteen eggs, you would buy a dozen and one eggs. In Britain you would not pay thirteen pence but rather a shilling and a penny. And still in a few places you would not measure up thirteen inches but rather a foot and an inch.
It should be noted that the other common number system in addition to the dozen is the score, which is 20. And you still can find traces of this in the language as well. This is why the teens are written differently from the higher numbers.
[Louis CK would like to have a word about this as well](https://youtu.be/k4LQ6shob3U?si=ZyBHghiQMZkuq0a8)
What about threeteen and fiveteen?
Why not onety-one and onety-two?
In some languages, like Chinese, they use a system where the numbers 11-99 do use the number ten in them. For example 11 is “ten one” and 99 is “nine ten nine”.
Surely they’d have been firsteen, seconteen?
[It’s a conspiracy to cover up the existence of eleventeen](https://www.youtube.com/watch?v=huyH3vlw3gI)
Today, base 10 math is pretty ubiquitous, but math wasn’t always base 10 and wasn’t always so standardized amongst ancient cultures. A lot of ancient people used base twelve, which was seen as better because it divided nicely in 2, 3, 4 and 6, whereas base 10 only divides in 2 and 5. Base 60 was also used sometimes, as evidenced by how hours and seconds, as it divides by 2, 3, 4, 6, 10, 15, 20 and 30.
So these special odd numbers are artefacts of these ancient math structures, and different languages, as mentionned by another commentor, have unique names going up to different numbers, e.g. French has special names up to 16. The math changed but people kept the old words because they were used to them.
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