a 24-hour-a-day city

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  • KHS

    Senior Member
    What I explain to my students is that "hour" here is functioning as an adjective (even though it is normally a noun - you can find this general type of structure referred to as a "compound noun"). In English, adjectives do not have a plural form, so nouns functioning as adjectives are not usually used in the plural.

    You would, however, make it plural if you said:
    New York is a city that functions 24 hours a day.

    Examples of other compound nouns that function in the same way:

    - a three-year-old child
    - a grocery store (NOT a *groceries store, even though it sells many kinds of groceries)
    - a bookcase (even though it holds many books)
    - a five-story building

    There are some exceptions to this pattern, but not many.
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