a sentence from the section "More examples" from cambridge.org:
(1) I was waiting at the airport when who should come along but Mr Pettigrew!

What does "when who should come along but Mr Pettigrew" mean?
Why do "when" and "who" stand together — why is "when who" grammatical?

my variant:
(2) I was waiting at the airport when someone should come along but Mr Pettigrew!
What is the difference between (1) and (2)?

  • 2
    Your variant doesn't make sense. When and who are only together because one is at the end of a phrase and the other starts a new one (the rhetorical question). Commented Mar 9 at 8:28
  • 2
    See this question Commented Mar 9 at 12:50
  • "Who should come along but X!" is an interesting use of but. Cf. the formulaic "Who of all people should come along but X!". The contrast is between anyone and everyone on the one hand and this particular person, X. That surprising coincidence of the speaker and X encountering each other explains the exclamation point.
    – TimR
    Commented Mar 9 at 13:13
  • It would actually be more appropriate to replace "who" with "no one", i.e. no one but Mr. Pettigrew should come along. That's the implied answer to the rhetorical "who" question. Commented Mar 17 at 11:15
  • “who … but” is an idiom equivalent to “you'll never believe (or guess) who …” Commented Apr 3 at 3:05

3 Answers 3


The rhetorical interrogative construction "when who should come along but X" establishes, with who a nearly infinite set of possible answers.

Compare this variant:

... when who, of all people, should come along but Mr Pettigrew!

It is that contrast between the implicit set "of all people" established by who ...but? and the particular individual, Mr Pettigrew, that is the element of surprise.

If you were telling a story about someone who came to a fork in the road, where there were only two possibilities, and the story had not set up any difference whatsoever between them, the construction would fall flat:

He came to a fork in the road, and which should he choose but the left one!

It's grammatical but absurd. There is no element of surprise.

someone, some individual, doesn't create the implicit set "of all people" like who does; nor does it create even the grammatical minimum of two. There is nothing to justify the rhetorical question, and neither is there a contrast between "someone" and "Mr Pettigrew" to justify the use of but.

I'd hire anyone but Mr Pettigrew.

I like everyone but Mr Pettigrew.

*I like someone but Mr Pettigrew. ungrammatical


When telling a story, a pattern like "Who should come along but X!" or "What do I see but Y!" is a way to add drama by posing a rhetorical question and then answering it. It usually indicates the speaker was surprised when the second part happened.

Another speaker might phrase (1) a bit differently:

(3) So I'm waiting at the airport. And who comes along but Mr Pettigrew!

"When" in (1) or "and" in (3) is just there to connect the two parts together.

Stated less dramatically:

(4) I was waiting at the airport. I was surprised to see Mr Pettigrew come along.

  • "Who should come along but X!" or "What do I see but Y!" — I can't find the meaning of "but" for these sentences in dictionaries. As far as I understand, "but" is a preposition here which does not mean "except". What word can "but" be replaced here with? Thanks.
    – Loviii
    Commented Mar 9 at 13:09
  • In my opinion "but" can be replaced with "except", but I'm not sure how common that is. It is an expression—hard to explain in terms of the literal meaning.
    – nschneid
    Commented Mar 9 at 17:57

I would say the structure "who should come along but [X]" is a rhetorical device used in English to emphasize the unexpected or coincidental arrival or appearance of someone. For the next question, using "someone" instead of "who" is not common and It reduces the element of surprise in such sentences.

  • 1
    And 'someone should come along but...' doesn't make sense. Commented Mar 9 at 12:48

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