As the title indicates, these three forms of words/phrases can be quite confusing to me sometimes. When should they be written as one word ("boilinghot"), when should they be written in two words ("boiling hot"), and when should they be written as a hyphenated compound ("boiling-hot")? Does it make any difference which is used? There are other terms like this, such as blackboard/black board/black-board; swingman/swing man/swing-man etc.


2 Answers 2


In the first place, I've never seen boilinghot used.

As for the other two, there are different times for different uses. Only use the hyphen when it is a compound adjective

Let's speak of why the sea is boiling hot.

Let's speak of the boiling-hot sea.

@drm65 illustrates how Google NGrams may be misapplied. Searching for a hyphenated expression will cause it to flat-line unless you put a space between the hyphen and the words: e.g. "boiling hot,boiling - hot"

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So while boiling hot appears to be used more often than boiling-hot, the latter's representation is not zero.

  • @Robusto: How do you know about the hyphen? It makes sense, but it doesn't seen to be in use. (See my edited answer.)
    – Daniel
    Jul 9, 2011 at 11:01
  • And, come to think of it, I don't think we use "boiling hot" in any context but as a compound adjective. In @Robusto's first example, "boiling hot" is a compound adjective.
    – Daniel
    Jul 9, 2011 at 11:27
  • 3
    @dmr65: Someone else clued me in to the hyphen trick a few months ago. BTW, why did you edit my answer to reflect a change in your answer? More polite would be to edit your own answer to reflect the change you made based on input from me, not to make my answer appear to reflect your changes. I'm rolling this back.
    – Robusto
    Jul 9, 2011 at 12:05

Two words is most common and appropriate: boiling hot. Unless it is used as an adjectival phrase; in which case the hyphen is appropriate for delineating the extent of the phrase. Though this Ngram illustrates that the hyphen is even then largely neglected.


As an aside: almost all phrases like this have a preferred form. Blackboard is the preferred form of that phrase:


And for swing man:


Edited: as @Robusto pointed out in his answer, I had inadvertently used the wrong syntax in my Ngram charts. Needless to say, I fixed the problem as soon as I realized it. Thanks again, Robusto!

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