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: 
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.
To add to the previous answers, the rule is Arabic numbers for horizontal writing: 

> 
> 123456789
> 
> 
> 

and Chinese numbers for vertical writing. 

> 
> 一  
> 
>  二  
> 
>  三  
> 
>  四  
> 
>  五 
> 
> 
>