"Eleven" is the modern English descendent of ancient Germanic word meaning one left (over ten).

"Twelve" was formed the same way.

English, like many other Germanic languages, retains traces of a base-12 number system. The most obvious instance is eleven and twelve which ought to be the first two numbers of the "teens" series. Their Old English forms, enleofan and twel(eo)f(an), are more transparent: "leave one" and "leave two."

"-teen" means plus ten and "-ty" means tens or times ten, so "fourteen" is four plus ten, and "twenty-four" is two tens and four. However, I don't know why numbers 11-19 have a different endianness from numbers 20-99.

The Old English word enleofan/endleofan/endlefan, which also mean "eleven" but can be literally translated as "the part left" or "the bit leftover," in this case meaning the bit left over ten.

Edit: "twelve" comes from the Old English twelf, which is ultimately a contraction of something like "twa-leofan", meaning something like "two left over" from ten. And while adding "-teen" on the end of 11 and 12 might make sense, the Romans didn't do it for all of the digits between 10 and 20 either. Latin does the opposite of Old English - it begins undecim (one more than ten), duodecim (two more than ten), but once you get to 18 its duodeviginti (two from 20), then undeviginti (one from twenty).

I read some nomadic tribes used base 12 systems for counting, because you can easily count to 12 using one hand if you count the joints of your fingers with your thumb. Supposedly the Babylonians adopted this system and that's why we have 12 hour days.

The linguistic explanations in this thread seem more convincing, but is there any connection?