Prefixing `一` to numbers


It seems to me that `千` is exceptional in having it prefixed with `一`.

  * As for `一`, `十`, `百`, they work together with `千` to describe the numbers from 1 to 9999, so they compete with one another, or are complimentary. There is no combination like:

> 一一, 一十, 一百, 十十, 十百 百百

  * As for `万`, `億`, `兆`, `京`, etc., they are multiplied by a number from 1 to 9999, expressed by the prefix using `一` `十`, `百` `千`, so when that prefix turns out to be 1, 10, 100, 1000, then they are combined, and there are combinations like:

> 一万, 十万, 百万, 千万, 一億, 十億, 百億, 千億, etc.

  * and there is no combination among themselves such as

> 万万, 万億, 億億, 万兆, 億兆, 兆兆, etc.

However, `千` is exceptional in that it is prefixed by `一` just when itself is
prefixed to the characters `万`, `億`, `兆`, `京`, etc.

> 千  
>  一千万  
>  一千億

Why is `千` prefixed with `一` in this case? Why is it not like the following?

> 千  
>  千万  
>  千億




I'm mostly guessing here, but I think it might be in order not to mistake them
for 三万 and 三億 when spoken. While せんまん and さんまん only differ by a vowel, いっせんまん
and さんまん are easy to distinguish.




I'm mostly guessing here, but I think it might be in order not to mistake them
for 三万 and 三億 when spoken. While せんまん and さんまん only differ by a vowel, いっせんまん
and さんまん are easy to distinguish.

