Languages are full of patterns. They're also full of words that break those patterns.
A listener named Dave Gee sent us a question about "eleven" and "twelve" which appear to belong in the pattern-breaking category.
"Why do these two number names not follow the pattern of every other number name?" Dave asked. "It seems like they should be 'oneteen' and 'twoteen.' After all, we have 'twenty-one' and 'twenty-two' and so on."
Dave rightly identifies a pattern here. A word like "nineteen" breaks down to "nine" and "teen," the latter of which goes back to the word "ten." The same pattern is clear in eighteen, seventeen, sixteen, and fourteen.
"Fifteen" and "thirteen" sound a little different, but they actually follow the pattern too. In "fifteen," the word "fif" is Old English for "five," so we have "five" and "ten"
In "thirteen," we're looking at a similar situation, this time with a twist. In Old English, "þreo" (pronounced "thray-oh") was the word for "three." So, thirteen started off as "þreo" plus ten.
What follows is a case of metathesis – the linguistic term for the swapping of two sounds. The equivalent of "thirteen" would've been "threeteen," However, the "r" and the vowel switched places over time. We also see this in "third" which used to be "thrid."
Now that we've followed the pattern all the way down, along come "twelve" and "eleven." These both go back to early Germanic compounds. The thing is, those compounds are no longer transparent to us.
In Old English, "eleven" would've been "enleofan." In this compound, "en" means "one," while the second part goes back to a Germanic word for "leftover." In other words, after you counted to ten, you'd have one leftover. "Twelve" follows the same pattern, with the Germanic "twa" (two) paired with that same root for "leftover."