ELI5: Why are the words "eleven" and "twelve," so different from the pattern of thirteen, fourteen, fifteen, etc?

This is one of those complicated etymology studies without an altogether clear answer.

Eleven and twelve are structures which show up in other northern-European languages, but the divide between using those forms and using the -teen form after twelve likely comes from the utility of twelve for trades-people.

Dozens and gross (144 or a dozen dozen) were preferred over tens for many trades for quite a long time due to their more convenient divisibility–evenly divisible by 2, 3, and 4–and this is the likely reason that the more continental European form survived in English. However, the evidence for any of it is very poor with conflicting theories even of the origins of the words.

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