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.)
More than likely it is borrowed from Chinese.

十 百 万 亿[億] 兆 京 

respectively:

10^1, 10^2, 10^4, 10^8, 10^16, 10^32 etc. (goes on until 10^4096 !)

I have seen 亿 and 兆 used in these definitions in modern Chinese and Korean, and certainly 億 in japanese for money e.g. 三億円 meaning 3 x 10^8 yen. 

This system of creating a new word whenever the length doubles seems natural, as it is the minimal set of words you need to represent any arbitrarily large number without repeating yourself. It also provides a more unique way to say a number. For example, in english we could say one million billion == 10^15 == one thousand trillion).

This has changed in modern china... 亿，兆，京 now sometimes mean smaller powers in different contexts for convenience. 兆 can mean a megabyte (10^6), e.g. 四十兆 => 40 Megabytes. Not sure if Japanese has this ambiguity, but 千 (10^3) certainly exists and doesn't fit the 'pure' traditional system.

In any case, I have never seen a split into blocks of 4 when writing a number down. But that is the logic that should be used when reading a number out. 

That is misleading though, since even larger numbers should be read by recursively bisecting at the highest 10^2^n which fits, and this can get a lot more abstract than simple powers of 10^4. To choose an unrealistic example:

1,000,020,000,304,567 =>

10000200 \* 10^8 + 00304567 =>

1000,0200 億 30,4567 =>

(1000 \* 10^4 + 0200) 億 + (30 \* 10^4 + 4567) =>

(1000 万 200) 億 (30 万 4567) =>

(一千 万 二百）億（三十 万 四千五百六十七）

Please note it is different than just separating with length 4 blocks: 10^12 does not have a unique name, but 10^16 does. So it really is unlike the western system. Blocks are only assigned a new name when they reach twice the length of the last named block. Rather than just 3,6,9,12,15, we have 4,8,16,32.

For negative powers of 10, the western style of 10^-3, 10^-6, 10^-9 is used... (equivalent to milli/micro/nano).