I often see the fraction ⅔ written with a hyphen, but I never see ½ written with one. Is it correct to have the hyphen in two-thirds, and if so why don't we write one-half?
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.)
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.
This is the same for "two-thirds".
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."