As there have been many good responses so far, I'd just like to add a little bit about the Latin number words in the teens up to seventeen:
undecim = one (plus) ten duodecim = two (plus) ten tredecim = three (plus) ten ... septemdecim = seven (plus) ten
Because of sound changes over many centuries, a Latin word like undecim was gradually "eroded" to once in Spanish and onze in French, in both of which there's practically nothing left that reveals the original word for ten. In Italian, on the other hand, undici still reveals the original words for one and ten pretty clearly. So we can't generalize about the words for eleven and twelve being special in the Romance languages.
There isn't anything special about the words for fifteen in Romance, unless you mean that in Spanish (and similarly in Portugues quince is the last of the sound-altered teen numbers that developed from Latin; from diez y seis onward, the numbers have been reformulated analytically.
In contrast, in the Germanic languages the words for eleven and twelve are special, meaning "one left (after ten)" and "two left (after ten)". Once again, because of changes in the language, the modern form eleven reveals practically nothing of the original word for "one" that it contains. Similarly, almost nobody would now connect the -lev- of eleven and the -lv- of twelve with the word "left," although the connection makes sense once it's pointed out.
Other instances of the "hidden" number one in English words are:
a, an (a book is one book, an apple is one apple) any (If you have any books, you have at least one) alone ("all one" = all by oneself) only ("one-ly") atone (to be "at one" with oneself) none (n[ot even] one)
---------------------- > I am a high school math teacher with a historical question. Bill Dunham >suggested I ask you. Why in both the Romance and Germanic languages are >eleven and twelve so different from the rest of the teens, and why is >fifteen in the Romance languages different from the rest of the teens? > > If you can help, I'd really appreciate it! > Bonnie Leitch > New Braunfels High School > New Braunfels, Texas >