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    • Don't buy more books than are necessary.
    • Don't buy more books than necessary.
    • We have three more doctors than are necessary.
    • We have three more doctors than necessary.

What is the difference between the two constructions?

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3 Answers 3

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Both structures are correct but, for the past fify years, "more than necessary" has come to be used more often than "more than is/are necessary". See Ngram below.

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Both forms are grammatical, there being no need for the verb 'is' or 'are'.

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  • Why is it? Curious what rule or principle makes ‘is’/‘are’ optional here.
    – CIFilter
    Aug 15, 2019 at 12:41
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    @CIFilter Simply because it is permissible in English to elide words that can be taken for granted. There are already other elided words in the original sentence, which might have been "Don't buy more books that it is necessary to buy".
    – WS2
    Aug 19, 2019 at 8:38
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I guess, it is the form of 'ellipsis' which omits the subject of the following clause, so it's used as 'than is necessary' rather than 'than it is necessary'. ...than (it) is necessary.

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