http://web.mit.edu/galib/www/FeatureList.html says, "You can use the types built-in to the library (bit-string, array, list, tree) or derive a chromosome based on your own objects."

I've sometimes seen "built in to the library".

Should there be a hyphen or not? Should it be "built-in to the library" or "built in to the library"?

  • 1
    'You can use the types built into the library ...' Dec 18, 2014 at 4:58
  • Perhaps it's using "built-in" as an adjective of some kind? i.e. 'you can use these data structures intrinsic to the library'
    – Einheri
    Dec 18, 2014 at 5:56
  • You can use the types built into the library or You can use the library's built-in types - alternative: You can use the native types of the library
    – mplungjan
    Dec 18, 2014 at 6:35
  • Use either "types built into the library" or "built-in types from the library" both of which should work fine.
    – Kris
    Dec 18, 2014 at 6:40
  • @user3109672 I've never encountered this sort of adjectival usage for built-in. Intrinsic to is a well-known collocation, but not built-in to . Dec 18, 2014 at 7:42

3 Answers 3


Using a hyphen is the wrongest thing to do here. Ideally, the sentence would sound something like this: "You can use the types built into the library". However, if the author meant that the types are somehow built TO the library, which is already grammatically wrong, he should at least have written, "types built in to the library", as a verb and a preposition are complementary.


Normally I see built-in as an adjective, whereas built in is used as a verb. ie: "the car has built-in air conditioning", and "the shelves are built into the wall"

  • (Downvote not mine) 'built into the wall' is a post-modifying participle phrase. As with many of these, there is ambiguity: it could mean eg 'The next step: the shelves are built into the wall' (more 'verby', addressing the process) or it could mean eg 'In John's flat, the shelves are built into the wall' (adjectival, describing the situation). Dec 18, 2014 at 7:48
  • Yeah I see what you mean about how it could either be a verb or adjective, depending on context. Either way, In the example given in the OP, I would not hyphenate the word, and turn it into built into Dec 18, 2014 at 7:50
  • No, 'it' can't be a verb. Or an adjective. It's a phrase (arguably a clause) – functioning verbally or adjectivally. It's 'a verbal' or 'an adjectival'. Dec 18, 2014 at 7:59
  • Oh, I see now, it's not a single word so it can't just be a verb/adjective. Dec 18, 2014 at 8:30
  • Not quite the whole story! 'Built into the wall' is certainly not a word / adjective, but in 'the aeroplanes land here, then take off again after refuelling', land is certainly a verb, but it can also be argued that the opposite, take off, should also be counted as a (multi-word) verb. Dec 18, 2014 at 11:25

What you're talking about here is a compound adjective. The rules regarding hyphens in compound adjectives are these:

If the compound adjective is right before the noun, use a hyphen:

  • Built-in types
  • A well-educated student
  • The fast-moving river

If the compound adjective directly follows a linking verb, don't use a hyphen:

  • The types are built in.
  • The student is well educated.
  • The river is fast moving.

However, if an adverb in a compound adjective ends in "y," don't use a hyphen regardless of where the compound adjective is:

  • A quickly moving river
  • The river is quickly moving.
  • A finely tuned watch
  • My watch is finely tuned.

Then there are some compound adjectives that are always hyphenated.

  • A state-of-the-art sound system
  • This sound system is state-of-the-art.

So in your example, you would not use a hyphen: "You can use the types [that are] built in to the library."

  • 1
    How can this be called an adjective? The postmodifier here is a participle phrase, 'built into the library'. As Mssrs chompchomp say. Dec 18, 2014 at 7:37

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