Numerals versus words, Part I: The basics

Once again, I have a reader to thank for suggesting today's subject. On the question of numbers, Pam writes, "I read to write them out if it's 1-99, and other places say write them out 1-9, with 10 going either way. I understand all the exceptions, it's just the general rule thing that I can seem to find an definitive answer."

Pam said a mouthful there. The truth is, there IS NO single, definitive rule. "It depends," while insufficient, is really the only correct answer here.

As with apostrophes (see here, here, and here), the numbers topic is perhaps best handled in a series. Today we'll look at the most basic guidelines, then follow up in the coming weeks with the most common exceptions.

According the The Chicago Manual of Style (15th Edition), the rule of thumb is this:

In nontechnical contexts, the following are spelled out: whole numbers from one through one hundred, round numbers, and any number beginning a sentence. For other numbers, numerals* are used. 

This general rule is followed by an entire chapter of exceptions and clarification.

On the other end of the spectrum is The Associated Press Stylebook (2004 Edition). AP doesn't start with a rule and then enumerate (very punny?) its many exceptions. Instead, it dives right into a case-by-case listing of when to and when NOT to spell out. Here are a few select examples that, together, form a summary:

•  "Spell out a numeral** at the beginning of a sentence....There is one exception--a numeral that identifies a calendar year."
• "Spell out casual expressions: A thousand times no!"
• "Use words or numerals according to an organization's practice: 3M, Twentieth Century Fund, Big Ten."
• "For ordinals...spell out first through ninth when they indicate sequence in time or location."
• No equal "For cardinals" rule exists; instead, a long list of separate entries for specific cases appears. However, it is generally safe to extrapolate the rule thus: Spell out one through nine when used casually.

Clearly these authorities approach the question very differently. A reasonable first step, then, is to determine the stylebook preference of the person or organization for whom you're writing. As an editor, my default answer is to follow the Chicago rule. But if you work for the media or in one of the many industries that is closely associated with the media, such as marketing or public relations, the AP rule is the authoritative answer.

*Chicago defines "numeral" only as an Arabic figure (ex: 1, 2, 3...).
**AP defines "numeral" as "a figure, letter, word or group of words expressing a number."