Today is November 11, 2011 — or, 11/11/11. It is a very “eleven” day.

But with the number 12, eleven captures an odd spot, linguistically. While all the other numbers (excluding zero to ten) seem to follow a formulaic pattern, these two — at first blush, at least — are outliers. In short, “11” is not “oneteen” and for that matter, “12” is not “twoteen.” What is going on here?

While you may think that the words emerged from a base-12 numbering system — think months of the year, hours in half a day, or inches in a foot — it turns out that this simply isn’t the case. “Eleven” is actually a base-10 term. The word “eleven” is derived from the Old English word “endleofan” (pronounced “end-lih-fen”) which itself comes from the Germanic “ainlif,” a compound word: “ain” means “one” and “lif” was a version of the word “left.” (The word “leave” has the same root.) Combined, “ainlif” means “one left.” Imagine a Germanic goat herder from the early Middle Ages counting his flock, putting them in units of ten — but missing his estimate and ending up with one left over. That last one is “ainlif” — “eleven.”

The word “twelve” follows a similar construct, from the terms “twelf” in Old English and “twalif” at its Germanic routs. Again, the word “ten” is assumed and the math still works: “eleven” is really “ten plus one” and “twelve” is “ten plus two.” There is nothing duodenary about their names.

For 13 to 19? “Teen” simply means “ten more than,” and of course, the prefix is self-explanatory. And no, we do not know why there is a linguistic split after twelve. Both the “teen” and “lif” terms developed at roughly the same time — probably around the year 900. The antiquity of these terms makes it impossible to determine the reason for certain.

Bonus fact : “Twelve plus one” is an anagram of “eleven plus two.” (And for whatever it is worth, “twoteen plus one” is an anagram of “oneteen plus two,” obviously.)

From the Archives: And, the 27th Letter of the Alphabet: The etymology of the term “ampersand.”

Related: This is Spinal Tap. If you do not know why this is related to the above, you really need to watch the movie. $9.99 to buy as a digital download; used VHS cassettes are available for a penny (plus shipping); and you can rent it digitally for $2.99. It has 397 reviews, fewer than 10% of which are for three or fewer stars.