When and how to use numbers in writing?

Question

When writing is it correct to use numerals (e.g. 1,2,3,) or spell out the number (e.g. one,two,three)?

Answer 1

The Associated Press spells out numbers below ten: one, two,...nine, 10, 11, etc.

The University of Chicago Manual of Style spells out numbers one through one hundred and uses numerals thereafter: one, eight, ninety-nine, 100, 101, etc.

In both cases, large round numbers are expressed in words: "one million" rather than "1,000,000."