- medieval monks and accountants start using Italian millione ("one thousand" + augmentative suffix) to mean 10^6 by the 1200s; this spreads to other languages
- Jehan Adam coins bymillion and trimillion to mean 10^12 and 10^18 in 1475
- Nicolas Chuquet extends this scale up to nonyllion (10^54), with every step being another six orders of magnitude (million, byllion, tryllion, quadrillion, quyllion, sixlion, septyllion, ottylion, nonyllion) in 1484. Note that in this period, it was common to put the digit separator every six digits instead of every three.
- Guillaume Budé refers to 10^9 as milliart in 1516, in a Latin text
- But in 1549, Jacques Pelletier du Mans uses milliard to mean 10^12, citing Budé as a source
- In the 1600s, people start putting digit separators every three digits. But some scientists and mathematicians define the numerical scale according to how digits are grouped, rather than the actual order of magnitude: thus, one billion becomes 10^9, one trillion becomes 10^12, etc, creating the short scale.
- "Milliard" is eventually added to the long scale, meaning 10^9 (in keeping with Budé's usage); the first published example is from 1676
- By 1729, the short-scale meaning of "billion" (10^9) has already crept into American usage
- This is in keeping with French usage at the time: in 1762, the Académie Française dictionary cites billion as meaning 10^9.
- By the early 19th century, France has almost completely converted to the short scale, and U.S. usage follows France; the long scale is referred to in some sources as "obsolete." But Britain is still using the long scale (and I assume Germany and most other European countries)
- Over the course of the 20th century, the long scale begins to become more influential in France, presumably due to the influence of continental usage; while the short scale becomes more influential in Britain, presumably due to the influence of American English. Notably the SI system very specifically uses unique prefixes that are the same across languages, to prevent confusion!
- In 1961, the French Government confirms that they're going to officially use the long scale from now on; in 1974, Britain officially switches over to the short scale, and many other English-speaking countries follow.
- In 1975, the terms "short scale" and "long scale" are actually coined, by mathematician Geneviève Guitel.
One reason large number names could be so unstable for so long is, of course, that outside specialized usage they are rare, and were even more rare before modern science and large modern monetary amounts became commonplace points of discussion. Wikipedia says "milliard" wasn't common in German until 1923, when bank notes had to be overstamped during Weimar-era hyperinflation.
As it currently stands, English, Indonesian, Hebrew, Russian, Turkish, and most varieties of Arabic use the short scale; continental Europe and most varieties of Spanish outside Europe use the long scale. A few countries use both, usually in different languages, like South African English (short scale) and Afrikaans (long scale) or Canadian English (short scale) and Canadian French (long scale) . Puerto Rico uses the short scale in economic and technical usage, but the long scale in publications aimed at export.
Notably some languages use neither, having their own names for large numbers--South Asian languages have the Indian numbering system, and Bhutan, Cambodia, and various East Asian languages also have their own numbering systems. Greek, exceptionally, uses a native calque of the short scale rather than a borrowing.
