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Words that are pluralized in the middle?

Why is the plural of "passerby" "passers-by" and not "passerbies"?

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    Related: Words that are pluralized in the middle? – RegDwigнt Jan 29 '11 at 22:50
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    Yes, this question seems like a duplicate of that one. (Or at least, this is subsumed by the other.) Also related, though not quite the same: possessives of such words. – PLL Jan 29 '11 at 23:02
  • I'm not certain that this question is a duplicate of the other one. This asks why such words are pluralized in this way, and the other question just asks for more such examples. – Tragicomic Jan 31 '11 at 13:12

I have no authorative source, but my common sense explanation is that the word is derived from the verb phrase to pass by something. As the noun-constructing suffix got attached to the verb part pass, making it passer, it is logical to chain the plural suffix -s after the first suffix. The by part is more of an additional particle for the word, I think it is even acceptable to write passers-by with a dash.

A weak point in this explanation is that other nouns derived from compound verbs don't build a plural in the middle (compare turnout, makeover). But those don't use a suffix to create the noun, so no noun-chaining can occur.

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    I believe the middle s only pops up in words whose first part cannot be a verb, only a noun: in "turnout" and "makeover", it is conceivable that they originated in phrases. You could say "the war will turn out fine": I cannot think of any usage of the noun "turn" that could result in a "turnout". In words like "makeover" and "let-down", it is even clearer that they come from verbal parts. If it existed, the imaginary word *passby would come from the verb "to pass", not the noun, so *passbys; in passer-, as you say, the suffix clearly makes it a noun. – Cerberus_Reinstate_Monica Jan 30 '11 at 4:51

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