Why should not put 一 before 千,百,十, but ok for 万?


I've heard that should not put 一 before 千,百,十, for example, we can say:

千円 or 百円 or 十円

but not:

一千円 or 一百円 or 一十円

But we can and must say:

一万円

So my question is: Is it correct? And why it's ok only for 万?




There are a couple of reasons for this. One part is that [万]{まん} is the the
point in the scale where things start looping (much like how in English we
group by sets of three 0s, Japanese does it by groups of 4). As such, it in
many ways behaves like a counter. Therefore, much like you wouldn't just say
[匹]{ひき} to refer to a single dog, you don't say [万]{まん} without prefacing it
with [一]{いち} to indicate a single group of 10,000.

This becomes even more obvious when you get into numbers over 99,999, where it
becomes

>   * [十万]{じゅうまん} - 100,000
>   * [百万]{ひゃくまん} - 1,000,000
>   * [千万]{せんまん} - 10,000,000
>   * [一億]{いちおく} - 100,000,000
>

As you can see, prefacing 十, 百, or 千 with 一 in these cases can get confusing
rather quickly. Hence the reason it's left off.

Also, coincidentally, this behavior is part of the reason why you'll sometimes
see larger numbers written out as a mix of Arabic numbers and kanji, such as

>   * 1万7000
>   * 30万円
>

etc.

When you get above [一万]{いちまん}, there _can_ be places where it's appropriate to
use 一 before 千 (e.g. [一千万]{いっせんまん}), but I'm not as well versed in that. That
said, we've got a very good explanation of it in this question here:
[Confusion about 一千億 and
一千兆](https://japanese.stackexchange.com/questions/14845/confusion-
about-%E4%B8%80%E5%8D%83%E5%84%84-and-%E4%B8%80%E5%8D%83%E5%85%86)




There are a couple of reasons for this. One part is that [万]{まん} is the the
point in the scale where things start looping (much like how in English we
group by sets of three 0s, Japanese does it by groups of 4). As such, it in
many ways behaves like a counter. Therefore, much like you wouldn't just say
[匹]{ひき} to refer to a single dog, you don't say [万]{まん} without prefacing it
with [一]{いち} to indicate a single group of 10,000.

This becomes even more obvious when you get into numbers over 99,999, where it
becomes

>   * [十万]{じゅうまん} - 100,000
>   * [百万]{ひゃくまん} - 1,000,000
>   * [千万]{せんまん} - 10,000,000
>   * [一億]{いちおく} - 100,000,000
>

As you can see, prefacing 十, 百, or 千 with 一 in these cases can get confusing
rather quickly. Hence the reason it's left off.

Also, coincidentally, this behavior is part of the reason why you'll sometimes
see larger numbers written out as a mix of Arabic numbers and kanji, such as

>   * 1万7000
>   * 30万円
>

etc.

When you get above [一万]{いちまん}, there _can_ be places where it's appropriate to
use 一 before 千 (e.g. [一千万]{いっせんまん}), but I'm not as well versed in that. That
said, we've got a very good explanation of it in this question here:
[Confusion about 一千億 and
一千兆](https://japanese.stackexchange.com/questions/14845/confusion-
about-%E4%B8%80%E5%8D%83%E5%84%84-and-%E4%B8%80%E5%8D%83%E5%85%86)

