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The plural form of cupful is cupfuls and cupsful? Shouldn't we be pluralizing the noun (cup) instead of the adjective?

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Cupful is a noun and follows the normal pluralisation rule, so the plural is cupfuls.

That said, cupsful does appear as an acceptable alternative in US English dictionaries https://www.merriam-webster.com/dictionary/cupsful

And the UK English Cambridge dictionary lists it as a US English alternative https://dictionary.cambridge.org/dictionary/english/cupsful

Obviously cupful is a compound of cup and full, so when separating the words you still pluralise the noun, e.g.: 'two cups full'

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    The punctuation of cupsful (in the US) follows the same style (despite it being a closed-form word) as attorneys general (as opposed to attorney generals). – Jason Bassford Jul 25 '18 at 16:43
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    @JasonBassford Did you mean pluralization? – chrylis -on strike- Jul 25 '18 at 17:39
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Cupful is a unit of measure. Two cupfuls doesn't imply two physical cups. You could measure two cupfuls of flour with one measuring cup, pouring the contents into a bowl. Two cups full of coffee does indicate two physical cups.

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    I'm pretty sure in the US nowadays you would just use cups when indicating a measurement. I've never seen a recipe that called for 2 cupfuls flour, only 2 cups flour (or 2 C flour, which I would still read as "two cups flour"). Of course this could be different for other varieties of English (and perhaps was different in the US in days of yore). – 1006a Jul 25 '18 at 17:33
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    True, recipes aren't usually written that way, but if you asked someone how much flour a recipe called for, the answer would be cupfuls, not cups full. – Literalman Jul 25 '18 at 19:22
  • And, yes, "cups" is more common than "cupfuls," but the question was about the plural of "cupful," rather than whether "cups" is more idiomatic nowadays, and I suppose it is. – Literalman Jul 27 '18 at 15:01
  • I already upvoted your answer because it's sort-of logical and might well be the historical reason why cupfuls developed. I just don't think the most common use-case today for "cupfuls" or "cupsful" is when using a multi-cup measuring cup, but rather when discussing several iterations of a single cupful. (Even in speech, if I'm cooking and someone asks "how much flour" I would just reply "two cups".) – 1006a Jul 27 '18 at 15:05
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So this is an interesting quirk of English.

At one point, "cupful" would've been written as two separate words (e.g. two cups full of water). As the phrase grew more common, people started to slur a little and smush it into one word -- cupful. Right as this linguistic smushing was taking place, people still remembered that the "head" of this word, the noun component that determines the bulk of the word's meaning, was "cup." So, people pluralized it as cupsful as a nod to cup having been the actual noun.

However, now that cupful as one word has been in the language for a while, people have forgotten that cupful was condensed from the phrase a cup full of ___. Cupful is its own entity, so it's treated like its own noun. Therefore, it's pluralized like any old noun: by tacking an s on the end.

An example of a word that's in the process of being smushed is "passerby." Plenty of people still think of it as the phrase "passer by" instead of as one word, so you'll see a hot debate brewing between the pluralization passersby and passerbys. In fact, my browser put a red squiggle under passerbys!

I learned this tidbit and some other cool stuff about linguistics from my boy Steven Pinker. This I learned in this book: https://stevenpinker.com/publications/words-and-rules , but I would highly recommend all of his books.

Hope this helped!

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    Thank you, it did. I pray to the Gods that passerbys don't get recognized as the right plural form of passer by anytime soon! – WorldGov Jul 25 '18 at 19:14
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    Surely if we are going to pluralize "passerby" as a normal English noun, it would be "passerbies", not "passerbys" ? (I am relieved to report my browser. Chrome, objects to both.) – Martin Bonner Jul 26 '18 at 12:55
  • Similarly, the plural of mother-in-law is mothers-in-law. – Scott Jul 27 '18 at 2:13

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