Let's count in Latin from one to twenty:

ūnus/ūna/ūnum, duo/duae/duo, trēs/tria, quattuor, quīnque, sex, septem, octō, novem, decem,

ūndecim, duodecim, tredecim, quattuordecim, quīndecim, sēdecim, septendecim, duodēvīgintī, ūndēvīgintī, vīgintī

As pointed out by symbiotech, "octodecim" and "novemdecim" were also used in Latin, but they didn't survive. On the other hand, as pointed out by martina, "dĕcem (et) sĕptem" was also a common form for "septemdĕcim".

In Attic Greek it was:

ΕΙΣ/ΜΙΑ/ΕΝ (heis/mia/en), ΔΥΟ (dúō), ΤΡΕΙΣ/ΤΡΙΑ (treis/tria), ΤΕΤΤΑΡΕΣ/ΤΕΤΤΑΡΑ (téttares/téttara), ΠΕΝΤΕ (pénte), ΕΞ (héx), ΕΠΤΑ (heptá), ΟΚΤΩ (oktṓ), ΕΝΝΕΑ (ennéa), ΔΕΚΑ (déka),

ΕΝΔΕΚΑ (héndeka), ΔΩΔΕΚΑ (dódeka), ΤΡΕΙΣΚΑΙΔΕΚΑ (treiskaídeka), ΤΕΤΤΑΡΕΣ ΚΑΙ ΔΕΚΑ (téttares kaì déka), ΠΕΝΤΕΚΑΙΔΕΚΑ (pentekaídeka), ΕΚΚΑΙΔΕΚΑ (hekkaídeka), ΕΠΤΑΚΑΙΔΕΚΑ (heptakaídeka), ΟΚΤΩΚΑΙΔΕΚΑ (oktōkaídeka), ΕΝΝΕΑΚΑΙΔΕΚΑ (enneakaídeka), ΕΙΚΟΣΙ(Ν) (eíkosi(n))

Now let's count in Italian

uno, due, tre, quattro, cinque, sei, sette, otto, nove, dieci,

undici, dodici, tredici, quattordici, quindici, sedici, diciassette, diciotto, diciannove, venti.

But numbers from eleven to twenty could also have been, just hypothetically of course (adding accents for clarity's sake):

diciùno, diciaddùe, diciattré, diciacquàttro, diciaccìnque, diciassèi, diciassètte, diciòtto, diciannòve, vénti


undici, dodici, tredici, quattordici, quindici, sedici, settèndici, ottòdici, novèndici, venti.

In Spanish it is:

uno, dos, tres, cuatro, cinco, seis, siete, ocho, nueve, diez,

once, doce, trece, catorce, quince, dieciseis, diecisiete, dieciocho, diecinueve, veinte

In Portuguese:

um, dois, três, quarto, cinco, seis, sete, oito, nove, dez,

onze, doze, treze, catorze, quinze, dezasseis, dezassete, dezoito, dezanove, vinte

In French:

un, deux,trois, quatre, cinq, six, sept, huit, neuf, dix,

onze, douze, treize, quatorze, quinze, seize, dix-sept, dix-huit, dix-neuf, vingt

Following martina's hint here are the number words from one to twenty in Romanian:

unu, doi, trei, patru, cinci, şase, şapte, opt, nouă, zece,

unsprezece, doisprezece, treisprezece, paisprezece, cincisprezece, şaisprezece, şaptesprezece, optsprezece, nouăsprezece, douăzeci

I find Latin, Italian, Spanish, Portuguese and French constructions for number words bizarre and inconsistent (whereas in Romanian it seems they are perfectly consistent - thanks martina for her hint - as well as in Ancient Greek). Is there any academic work on the history of number words in Latin/Italian/Spanish/Portuguese/French where the origin of number words from eleven to nineteen is tracked down, documented, explained, discussed?