I looked at this book of Gerbert of Aurillac's writings, where I found his explanation of how to use an abacus. It's just great, I think (if you like numbers!).

In hôc volumine operum Gerberti quaesivi in quo modus enumerandi per abacum tractatur, ut feliciter inveni. Quam magna est haec fons, ut credo (si numeri tibi placent)!N. Bubnov (ed.),

Gerberti postea Silvestri II papae Opera Mathematica (972-1003), Berolini 1899 (

http://www.archive.org/details/gerbertiposteas00sylvgoog)

Commentarius, p.249 // pagina ducenti quadraginta novem, wrote:Nam sicut unitas significat X in decenis et C in centenis, sic etiam mille in millenis et decem millia in decenis millenis, nec non centum millia in centenis millenis. Binarius vero, quomodo XX in decenis et CC in centenis, sic etiam duo millia in millenis, et XX millia in decenis millenis, et ducenta millia in centenis millenis, et cetera ad hunc modum.

For just as one signifies 10 in the 10s [column] and 100 in the 100s, so too 1000 in the 1000s and 10000 in the 10000s, as too 100000 in the 100000s. Two of course, just as 20 in the 10s and 200 in the 100s, so also means 2000 in the 1000s, 20000 in the 10000s and 200000 in the 100000s and so on in this way.

"A one" = "unitas"; "a two" = "binarius"; "a three" = "ternarius"; "a four" = "quaternarius"; "a five" = "quinarius"; "senarius" = "a six"; "a seven" = "septenarius"; "an eight" = "octonarius"; "a nine" = "novenarius".

To count in tens, in hundreds, in thousands is "In decenis, in centenis, in millenis numerare".

Then "decem millia" (10000), "centum millia" (100000), "mille millia" (1000000), "decies mille millia" (10000000) [at

non decem mille millia, quia singulis numeri est mille], "centies mille millia" (100000000), "millies mille millia" (1000000000), "decies millies mille millia" (10000000000), "centies millies mille millia" (100000000000), "mille millies mille millia" (1000000000000), "decies mille millies mille millia" (10000000000000) and so on // et caetera!

Here is Pliny the Elder:

Plinius Secundus, Naturalis Historia, liber tricesimus tertius, capitulum quadraginta septem, wrote:Non erat apud antiquos numerus ultra centum milia; itaque et hodie multiplicantur haec, ut decies centena aut saepius dicantur.

But here is Livy:

Livius, Ab Urbe Conditâ, liber tricesimus sextus, capitulum quattuor, wrote:Carthaginienses tritici modium sexcenta [600000], hordei quingenta milia [500000] ad exercitum, dimidium eius Romam apportaturos polliciti sunt: id ut ab se munus Romani acciperent, petere sese. et classein [suorum] suo sumptu comparaturos; et stipendium, quod pluribus pensionibus in multos annos deberent, praesens omne daturos. Masinissae legati quingenta milia [500000] modium tritici, trecenta [300000] hordei ad exercitum in Graeciam, Romam trecenta milia [300000] modium tritici, ducenta quinquaginta [250000] hordei ; equites quingentos , elephantos viginti regem ad M. Acilium consulem missurum.

I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.