Chicago Manual of Style, paragraph 5.92 has a sentence that starts with the following: "For instance, pregnancy lasts nine months but is a nine‐month pregnancy, . . .", which is then followed by another example.

How do you interpret "but is" here? It doesn't make sense to me. Did they mean "is" and erroneously placed "but" in there, or am I missing something?

  • The clue is the italics. It means, "Write the phrase 'pregnancy lasts nine months' without a hyphen between nine and months, but write the phrase 'nine-month pregnancy' with a hypen."
    – user105719
    Feb 7, 2020 at 9:47

2 Answers 2


The noun pregnancy in phrase A behaves according to [the complete sentence] A, but is (defined as) B (nominalised using the phrasal adjective 'nine-month', not 'nine months').

The logic behind that 'but' is: '(it lasts) nine months' but '(is) nine-month' as a phrasal adjective (singular, with a hyphen) defining a noun. The whole expression suggests to compare the two phrases in italics for finding that difference.

  • If they mean "this is done THIS way (plural, no hyphens) but THAT is done THAT way (no plural, hyphens present)", then is (intuitively) looks and sounds like it doesn't belong there when it immediately follows but. But am I describing what they tried to convey correctly? (THIS is like this but THAT is like that.)
    – bp2017
    Feb 7, 2020 at 19:08
  • 1
    Technically, but connects the verbs lasts and is. The same thing (pregnancy) lasts [how long?] nine months but is [what?] a nine-month thing [if we build a noun phrase with an adjective to define it].
    – Alex_ander
    Feb 7, 2020 at 20:04

It's how they explain to you that you need to usa a hyphen when talking about nine months as an adjective. It's like "My son is five years old" vs. "It's a five-year-old cat".

So they're basically explaining how a phrasal adjective behaves in such sentences.

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