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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.


2 Answers 2


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, 2011 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, 2011 at 12:18
  • It's appalling to admit it here, but my sympathies go with the astronomers.
    – pavium
    Jul 16, 2011 at 12:25

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