For title case that does not capitalize articles, conjunctions, and small prepositions, how should one capitalize compound verbs and idiomatic phrases containing one of these elements?

For example,

when in doubt, smash it

Following the title case I described, it could be written as:

When in Doubt, Smash It

However, the "in" is part of the idiom "in doubt" rather than being used alone. Using that logic, it might be more appropriate to write it as:

When In Doubt, Smash It

Which way is most appropriate? Do any style guides address this problem or similar ones?

  • 1
    Style guides are only guides. You can write it how you like as long as you're consistent.
    – Mynamite
    Aug 11, 2014 at 21:28
  • “Being an idiom” is a red herring. What matters is whether it is a phrasal verb or a preposition.
    – tchrist
    Aug 12, 2014 at 0:17
  • Don'tGoBreakingMyHeart. Aug 12, 2014 at 0:21

1 Answer 1


The various style guides that I'm familiar don't alter their guidelines for title capitalization depending on whether an included preposition (such as in) is part of an idiom or not.

That's not to say that none of them ever alter their preferences in special instances—such as when the preposition is more closely associated with the verb than with the associated prepositional phrase, as discussed in earlier Q&A's on this site (How to capitalize "get out of" in a title according to CMOS? and Capitalising "for" depending on the usage in the title?).

Style guides do the darnedest things, so I wouldn't be stunned to learn that some guide somewhere does treat prepositions in idiomatic phrases differently from other prepositions playing the same role but not in idiomatic phrases. From a reader's perspective, though, I don't see any advantage to be gained from making such a distinction, which is probably why most style guides do not endorse treating prepositions in idioms as a special case.

  • 1
    It actually has to do with whether in is a preposition or a particle. “Being in doubt” has in as a preposition whose object is doubt so gets no cap, while “smashing something in” has in acting as a separable particle in the phrasal verb smash in, and so would get one.
    – tchrist
    Aug 12, 2014 at 0:17

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