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I often see the fraction 2/3 written with a hyphen, but I never see 1/2 written with one. Is it correct to have the hyphen in "two-thirds", and if so why don't we write "one-half"?

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+1. "One half" often is written with a hyphen -- a Google search for "one half" on NYTimes.com finds plenty of instances with a hyphen and plenty of instances with a space -- but the frequency difference is interesting nonetheless. –  ruakh Jun 26 '12 at 15:09
    
Maybe people are so used to seeing "a half" without a hyphen that they (incorrectly) leave it out of "one-half". –  Peter Shor Jun 26 '12 at 19:32
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4 Answers

up vote 8 down vote accepted

The Chicago Manual of Style has these guidelines:

For compounds formed with fractions:

  • The noun form is open (a half hour)
  • The adjective form is hyphenated (a half-hour session)

For simple fractions:

  • Hyphenated in noun, adjective, and adverb forms, except when second element is already hyphenated (one-half; one and three-quarters; one twenty-fifth)

So "1/2" should always be written out as one-half. (Unless it's in a sentence like "one half of a perfect pair," in which case it's not a fraction.)

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One half need not be hyphenated when used as a noun; however, it must be hyphenated when used as an adjective:

 1. I am entitled to one half of the pizza.
 2. I have a one-half interest in the pizza.
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There's nothing wrong with your answer, but an answer citing a source gives the same rule. (Admittedly, it does not supply examples). –  virmaior Feb 5 at 2:45
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It is correct to have the hyphen. I am not quite sure why the people that you see write "one half" do not write "one-half" but they should!

Searching for "one half" at dictionary.reference.com does not return any results, but searching for "one-half" does.

One-half dictionary.reference.com

This is the same for "two-thirds".

Two-thirds dictionary.reference.com

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This made me think of when you write a check: "One hundred and twenty-two..." but there is no hyphen in the first portion. See Grammarly for examples of when to hyphenate: "When writing a compound number...we use a hyphen in between them. This applies to any number between 21 and 99. Numbers higher than ninety-nine don’t require a hyphen. This rule applies even if these numbers are preceded by other numbers. The rule also applies if a number between 21 and 99 is being used as an adjective."

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Sorry...since this question was already answered, I meant to post this as a comment to another answer but had it down in the "answer" section while I edited it. I guess I'll leave it where it is. –  hotshot309 Feb 26 at 22:36
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There is nothing wrong with adding an additional answer to an already 'accepted' question, so long as with your addition the question as a whole is a better resource. –  DougM Feb 26 at 22:58
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