There is THIRty, then there is THIRteen. So if there is TWENty, why is 12 twelve instead of "twenteen?"

English is multiple other languages in a trench coat, we stole that number from old German.

English takes words from many places in history. Nobody chooses these things, they just evolve over time. You’ll notice 11 is not “oneteen” either. We just have a more convenient word for it ¯\_(ツ)_/¯

twelve came from proto-German meaning "two left"; as in 12 is 10 with two left over. eleven is 'ein (one) left'

Pretty sure other European languages also have unique names for 11 and 12, likely because things were usually measured in twelves. Eg twelve inches to the foir, twelve pence to the shilling. Twelve is a good number for mental arithmetic because it can be divided evenly by 2, 3, 4 and 6, and simply by 8 and 9.

English is hardly the only language that does this. Spanish, for example, basically does this up to fifteen- once, doce, trece, catorce, quince - then switches to diez-y-seis, diez-y-siete, etc. Don't even get started on the vagaries of French numbering (four twenties, ten, and nine is how they say 99). The answer is...that's just how the language evolved. Languages (other than Esperanto or fake languages for books/TV/movies like Klingon or Dothraki) are not developed by a person or committee writing down logical rules. Languages are a naturally-evolving negotiation between millions of people across time and space attempting to communicate. They are shaped by everyday practical utility, by trade, contact, and conflict between cultures, by technology, and every other human activity.

And then we just scrap twelve and call things a dozen.

When I taught my kids to count, I told them it was tenty-one, tenty-two etc but that everyone else called them something different. Like how now I’m teaching them circles with tau instead of pi. I don’t know if it helps but it certainly presents a cleaner model.

The numbers in German, Dutch, and Scandinavian languages also follow this patter. Not until you reach "13" does it become essentially "ten and three" or some variant.

These inconsistencies happen in many languages. In French, there is a consistent pattern of words for multiples of 10 up through 60, but then it gets weird. 70 is "sixty-ten", 80 is "four-twenties" and 90 is "four-twenties-ten". I have heard that these inconsistencies are leftover pieces of the base-20 number system used by celtic cultures in western Europe.

Twelve is more like twoteen than twenteen

It’s not just English, it’s like this in German, similar in French (starting from 17) and many none

Culturally, we used to use some 12-based counting systems for measuring things, because it's divisible by 2,3,4 and 6 which is pretty useful. Old British money used to be all 12 pennies in a shilling, with sixpence and thre'penny (three penny) bits, and 240 pennies in an old £1 note. My point is, aside from the origins of the words themselves, having unique names for 11 and 12 made a lot of sense for a long time. Now everything is metric and decimalised they stand out as breaking the pattern, but that's because they're part of an older pattern.

For the same reason it's venti/dodici in Italian and veinte/doce in Spanish and zwölf/zwanzig in German. Because that's how it is.

It’s not unheard of for a language to use different language for eleven and twelve compared to the rest of the teens. Greek does it differently as well: eleven is en-deka, twelve is do-deka, and then the rest of the teens change it up - thirteen is deka-tria, fourteen is deka-tesera, and all of the rest of the teens follow that pattern of deka- instead of -deka.

It’s all fucked up for historical reasons. Fixing it would significantly boost kids mathematics learning across the English speaking world.

This is one of the few cases where English isn't the worst offender. The French word for 96 translates to "four twenties and sixteen".

At one point people would count on their knuckles. Four fingers three segments meant you could go to twelve. As it was common to use this method word usage conformed. Another post here describes the ten and one left over and ten and two left over.  I have always found this interesting because in French they go all the way up to sixteen before they go ten-seven ten-eight ten-nine. We on the other hand start at thirteen. I would be interested in finding a practical explantation how the French adopted another four numbers.

Listen, I hear you, but I refuse to answer until someone holds French and Danish accountable for their counting crimes.

You're lucky. This crap goes up to sixteen in french.

Because 12 is a natural counting base on one's digits (finger bones). People without school education didn't care about decimal system, they used dozens to count things.

Proto-Germanic formed the numbers 11 and 12 from a ten-count numbering system. 11 was “ainalif” and 12 was “twalif” which meant “one left over” and “two left over” respectively. 12 was the number you had if pulled out a group of 10 and had two left over. Regular sound changes morphed those words to “eleven” and “twelve” over the last ~2000 years. 13 - 19 used the form “3-10, 4-10, 5-10 (“þritehun, fedurtehun, fimftehun”) etc., which English still does. We don’t know why Proto-Germanic treated 11 and 12 differently, but it’s not the only European language that does that. Multiples of ten were literally “two tens (twai tigiwiz), three tens (þrīz tigiwiz), four tens (fedwōr tigiwiz), etc.” “Tigiwiz” (literally “tens”) got shortened in the various Germanic languages, to “-ty” in the case of English.

Don’t use logic and reason with English. This is one of those “because it is” things. It’s an inherited and very old part of the language that doesn’t make sense any longer.

Mostly because English is figuratively the jack sparrow of languages. It waltzes around, mostly drunk, plundering whatever it feels like even from non-latin based languages like itself.