Question: As I understand, Japanese numbers are divided into blocks of four, so while we would think of the number 89123889 as 89,123,889, in Japan they would think of it as 8912,3889 (八千九百十二万三千八百八十九). So English uses thousands and Japanese uses tens of thousands.

Is there any particular reason for this? Not that it matters either way, I'm just wondering at what point either culture chose to delimit numbers by thousands, or tens of thousands.

If you reply with Japanese, please use hiragana and furigana only, except for number kanji.

Answer: 
The following is a summary of [this Wikipedia article](http://ja.wikipedia.org/wiki/%E5%91%BD%E6%95%B0%E6%B3%95).

A math book called 塵劫記【じんこうき】, published in 1627, was the first book that described (and probably defined) how to count large numbers in Japanese.

In the [first edition of the book](http://dbr.library.tohoku.ac.jp/infolib/user_contents/wasan/l/j001/06/j001060007l.png), there was no "4-digit grouping" as we know today, at least for relatively small numbers (smaller than 1 極【ごく】). A different kanji was assigned to each digit. 104 was 万, 105 was 億, 106 was 兆 ... and so on, until it reached 1 極, which was only(?) 1015.

Soon after that, the 4-digit grouping was introduced in a revised edition of 塵劫記 published in 1631, and 1 極 became 1048. In this edition, 8-digit grouping was still used for numbers larger than 1 極 (100000000 極 = 1 恒河沙【こうがしゃ】, 100000000 恒河沙 = 1 阿僧祇【あそうぎ】, and so on)

In the [year 1634 edition](http://mahoroba.lib.nara-wu.ac.jp/y05/html/380/l/p006.html), the 8-digit grouping was completely removed, and the simple 4-digit grouping system after 万(=104) remained. This is exactly how the Japanese language count large numbers today.

Before 塵劫記, Japanese had a relatively [small vocabulary](http://www.sf.airnet.ne.jp/ts/language/number/ancient_japanesej.html) for big numbers, and 万【よろづ】 (=104) seems to have been the largest unit. 八百万【やおよろづ】 meant "countless."

This 4-digit loop was borrowed from Chinese numeric system. Korea also uses a similar system. However I couldn't find why and when the Chinese language started to use the 4-digit scale instead of 3.

(By the way, I was surprised to find that English-speaking people used [long (6-digit) scale](http://en.wikipedia.org/wiki/Long_and_short_scales) until relatively recently.)
Numbers in English and most "western" languages are still influenced by [Roman numerals](http://en.wikipedia.org/wiki/Roman_numerals), where 1000 = M = *mille* was the largest number that had its own, non-compound name.

Japanese took its numerals originally [from Chinese](http://en.wikipedia.org/wiki/Chinese_numerals#Large_numbers), where there is a separate character for "ten thousand". It also has characters for larger numbers, but groupings of 5 or more are probably less convenient as they are harder to recognize quickly.

However, neither system historically came into being fully complete and used consistently. Both changed through the times and there were conflicting usages for a long time until everyone converged on a standard.