English does some inconsistent tricks when it comes to numbers. After ten is “eleven” and “twelve”, but then a sequence of “-teens”. So why isn’t “eleven” called something like “oneteen” and twelve “twoteen”?
You can find the first clue by checking out the various non-metric measurement systems used (also inconsistently) in English-speaking countries. One foot, for example, comprises not ten inches but twelve. Twelve is a more convenient base for linear measurement when you’re making things by hand, because you can divide lengths by two, three, and four without getting into fractions or decimal places. A skilled carpenter can come very, very close to cutting a board in half or thirds just by eye. And if what you’re building doesn’t need the kind of precision that’s only really existed in for about the past century, or a bit longer, what you can do by eye is often close enough.
The old English monetary system was (partially) based on 12 as well; there were 12 pence in a shilling. And we still count many items by the dozen and/or the gross. But at the same time that base 12 was more convenient for some things, base 10 was good for other things, particularly given ten fingers to count on. Although some cultures have used bases other than 10 and also counted on extremities — if you use all 10 fingers and count each hand, you’ve got base 12.
When you’re dealing with both tens and twelves, the two numbers in between — that would be 11 and 12, obviously — are special. They’re “ten and one” and “ten and two.” In fact that’s exactly what “eleven” means. In Old English it was “endleofon”, a compound word for “one left over.” “Twelve” is the same, the Old English version was “tuoelf”, and meant “two left over.”
That raises a question, though. Once you pass ten, and you call the next number “one left over” and the next “two left over”, why switch at that point to “thirteen”, which means “ten and three”? (The “-teen” in thirteen is just a variation of “ten” in the same way that “thir-“ comes from “three”.)
There probably isn’t any particular reason. For one thing, other languages approached the issue differently; in French each number up to 17 has its own unique word. Germanic languages like German and Dutch use the same approach as English, while Romance languages like French and Italian don’t.
Then when you get into multiples of ten, like twenty and thirty, you’re faced with another issue if numbers higher than ten have their own names. That is, if nine tens is “ninety”, why can’t you call eleven tens “eleventy”? The answer might be that “twelvety” is too hard to say, and “eleventy,” or 110, isn’t a number anybody has a routine use for. “Eleventy” has appeared here and there: “She got a rug remnant at a bargain sale downtown for eleventy-nine cents” (1905), but it’s almost always just supposed to be funny. “The number of copies made in American offices grew from about 20 million in 1955..to approximately eleventy zillion today.”
And don’t forget that “dozen” means 12 and “gross” means 144, which is 12 twelves. But we don’t have a fraction of the handy terms the Sumerians probably had 5,000 years ago. They used base 60!
