# 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:

but not:

But we can and must say:

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