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When is it necessary to use a hyphen in writing a compound word?

What is correct, deep-sky object or deep sky object? Does common usage trump grammar?

Or, alternatively, if they are both correct, which one should be preferred?

Inconclusive discussion on Wikipedia.

marked as duplicate by Daniel, Alain Pannetier Φ, kiamlaluno, user11550, Matt E. Эллен Aug 28 '12 at 9:55

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Googling around, I find that the non-hyphenated form is considerably more prevalent. But as for grammar, the hyphenated form is more correct, since deep is modifying sky, not object.

Deep-sky object = [Deep sky] [object]

The non-hyphenated form would be correct if both deep and sky were modifying object:

Deep sky object = [Deep] [sky object] or [Sky object which is also deep]

The best clincher on this I could come up with is that Ngrams, which only analyzes books, has no instances of deep sky object; only of deep-sky object:


Granted, there are not many hits, but it's still a lot compared with zero.

If I had to choose, I would choose the hyphenated form.

  • 1
    I like statistical evidence as much as the next guy, but the Ngram just demonstrates that editors employed by printing houses have a good grasp of grammar. mental note: Gotta learn to use 'ngrams' – pavium Jul 16 '11 at 12:35

The inconclusive discussion is a Wikipedia article about a game by that name where the article was apparently highjacked by astronomers - what would they know?

Both spellings are used in the article. But there doesn't seem to be confusion.

The answer to your question might depend on who you're communicating with: astronomers or grammarians.

But I suspect both parties would say "It doesn't matter"

  • 3
    It's true that there is little confusion, but there is a right answer as far as English goes. – Daniel Jul 16 '11 at 12:18
  • It's appalling to admit it here, but my sympathies go with the astronomers. – pavium Jul 16 '11 at 12:25

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