Q From Tony: My wife is Japanese and we were discussing why Japanese have 10 different names for the same number. I asked why? She returned with a really tough one. Why are eleven and twelve so named? Why not eleventeen and twelveteen? She had a point. Where did those words originate?
A The ending -teen is just an old form of ten, so that sixteen is “six-ten” or “six plus ten”. If you were following the rule of such numbers strictly, you ought to count oneteen, twoteen, threeteen, fourteen, and so on. (Thirteen is actually a modified form of threeteen, a word that existed at one time, though written as threteen.)
As you say, eleven and twelve don’t fit this neat system. That’s probably because people a thousand years ago didn’t necessarily think in tens all the time, but often preferred twelves. The posh name for it is the duodecimal system; think of 12 inches to the foot or 12 pence to the shilling in old British money and the way that items even now can be counted out by dozens or by the gross. It may also in part be tied up with the idea of the dual number, in which things were classed as one, two, or many (singular, dual, plural); Old English had a dual number, as do other languages. So ten plus one and ten plus two were special numbers, not quite in the same situation as ten plus three or the higher examples.
The oldest form of eleven in English is endleofan (which appears in King Alfred’s translation from the Latin of Bede’s Ecclesiastical History). It’s made up of two words that mean “one left over”, that is, after one has counted ten, there’s one remaining. Similarly twelve is from Old English words that meant “two left over”. So why didn’t people talk about “three left over” and so on? It seems that once they had got past the magic twelve, people swapped to counting in a more direct way.
Note, though, that this pattern isn’t by any means universal even among European languages. Though German and Dutch match it, French and the other Romance tongues don’t (French, for example, has unique words up as far as seventeen). The pattern seems to be peculiar to the Germanic languages.