Numbers

On Friday, I helped a friend edit some copy for a give-away postcard. She asked, "What's the rule about spelling out numbers?" There's no simple answer, because different style manuals suggest different things. The two rules I come across most are these:

1. Spell out numbers from zero to ten. Anything above that, use numbers.
2. Spell out numbers from zero to ninety nine. Again, above that, use numbers.

I see the first rule used more often than the second. However, there are some variations on this. For instance, it's usually better to write two million than 2,000,000. (Unless it's the receiving end of money. Then people want to see all those zeros!) The most common rule here seems to be that round numbers are often spelled out.