How would you capitalize a word in a book/article title when that word has an en dash in the middle of it? For instance, should "protein-protein interaction" be capitalized as "Protein-protein Interaction" or "Protein-Protein Interaction"?

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    I think this is a style issue that does not have a single, definite rule; when I worked for a company that put together large catalogs, we were required to only capitalize the first word and never the second word.
    – Kosmonaut
    May 25, 2011 at 15:07
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    @Kosmonaut, do you mean you capitalized the first hyphenated word but not the second and capitalized the remaining significant words (as in the poster's example), or do you mean you had something like "Particle-particle interactions"? That's a valid style too; I was taking the capital "I" as a constraint of the question. @Tamás, if I'm reading you wrong, please let us know. May 25, 2011 at 16:53
  • @Monica Cellio: I take it for granted that I have to capitalize every "significant" word in the title according to the usual capitalization rules, so yes, consider the capital I in "Interactions" as a constraint :)
    – Tamás
    May 25, 2011 at 17:47
  • I would capitalize both instances of "protein" because the en-dash is functioning as the word "to," i.e., "Protein-to-Protein." I suspect that this was actually an overgeneralization of the rule for capitalizing with hyphenated prefixes.
    – The Raven
    May 25, 2011 at 18:24
  • @Monica Cellio: I'm saying that, if the company were selling a product with that name, the policy would be to only write it as "Protein-protein Interaction". I'm not advocating for everyone to do it that way, only that this was the policy for a major US company.
    – Kosmonaut
    May 25, 2011 at 19:39

1 Answer 1


Capitalize each word that is a word on its own, as in "Protein–Protein Interaction". If the hyphen adds a prefix that doesn't stand on its own, like "Non-protein Elements", then capitalize the prefix. But I've seen it both ways.

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