Numbers are handled differently depending on whether you’re using the Associated Press (AP) Stylebook style guide or the Chicago Manual of Style style guide.
When discussing numbers, you will need to know the following:
Cardinal numbers: one, 7, forty-one, one hundred nine, 852, three thousand sixty-one
Ordinal numbers: 1st, seventh, 41st, 109th, eight hundred fifty-second, 3,061st
Arabic numerals: 1, 7, 41, 109, 852, 3,061
Roman numerals: I, VII, XLI, CIX, DCCCLII, MMMLXI
Basic Number Rules (for Nontechnical Copy)
AP (p. 203):
Spell out whole numbers up to (and including) nine (e.g., zero, one, 10, 96, 104).
Spell out casual expressions: “A picture is worth a thousand words, but a really good one is worth a thousand dollars.”
Chicago (9.2-4, 9.8):
Spell out whole numbers up to (and including) one hundred (e.g., zero, one, ten, ninety-six, 104).
Spell out whole numbers up to (and including) one hundred when followed by “hundred,” “thousand,” “hundred thousand,” “million,” “billion,” and so on (e.g., eight hundred, 12,908, three hundred thousand, twenty-seven trillion).
Alternative rule: Spell out whole numbers up to (and including) nine, and use numerals for the rest. You have a choice.
Numbers Beginning a Sentence
AP (p. 202): Spell out numbers that begin a sentence unless it begins with a year (e.g., “Twelve drummers,” “The 10 lords a-leaping,” “2011′s quota for off-season holiday references has been filled.”).
Chicago (9.5): Always spell out numbers that begin a sentence, or reword to avoid unwieldiness. Well, if you think that “Nineteen ninety-one” looks more awesome than “The year 1991,” then go right ahead.
Note: There is no “and” when you spell out whole numbers (e.g., “one hundred one Dalmatians,” not “one hundred and one Dalmatians”).
AP (p. 202): Spell out ordinal numbers up to (and including) “ninth” when indicating sequence in time or location (e.g., first kiss, 11th hour) but not when indicating sequence in naming conventions (usually geographic, military, or political, e.g., 9th U.S. Circuit Court of Appeals).
Chicago (9.6): Spell out ordinal numbers up to (and including) “hundredth” (e.g., second, sixty-first, 333rd, 1,024th).
A Word About Consistency
AP (p. 203): If you’re juggling a bunch of numbers within the same sentence, stick to the rules as stated.
Chicago (9.7): If you’re juggling a bunch of numbers within the same paragraph or series of paragraphs, be flexible with the number style if doing so will improve clarity and comprehension. For example, use one number style for items in one category and another style for another category: “I read four books with more than 400 pages, sixty books with more than 100 pages, and a hundred articles with less than 4 pages.”
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