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Why are year, pound and mile in the singular form in the phrases below?

  • five-year-old children
  • 20 pound note
  • 10 mile run

Is that because they're acting as adjectives, which are always invariable in English?

Is it incorrect to say...

  • five-years-old children?
  • 20 pounds note?
  • 10 miles run?
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3 Answers 3

up vote 23 down vote accepted

Those are called compound and hyphenated compound adjectives. And adjectives don't have plural forms.

Additional examples

three-storey building (three-stories bulding)

four-wheel drive (four-wheels drive)

32-bit processor (32-bits processor)

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further, when "I am five years old." is used, it is non-hyphenated, and the years old is further modifying on the five (could also be "five feet tall" etc); leaving you with a primary structure of "I am five | Five am I". In other words, the examples in the original post cannot be switched structurally and retain their meaning –  mfg Aug 19 '10 at 12:15
Just in case someone notices that you can also say "she is a five-year-old", this is an example of an adjectival noun: en.wikipedia.org/wiki/Adjectival_noun_(noun%29 . As for the "I am 5" thing, that works for age, but doesn't work for "I am 6 feet tall" (≠ "I am 6"). It is a little more complicated than that. –  Kosmonaut Aug 19 '10 at 12:29

On the other hand, you could describe a successful tennis player equally as "a five-times winner of the Australian Open" and "a five-time winner of the Australian Open". And the former British government minister John Prescott, who was nicknamed 'Two Jags' (because he had a Jaguar as an official vehicle in addition to the Jaguar he owned), was sometimes referred to as 'Two-Jags Prescott', not 'Two-Jag Prescott'. So the general rule is not absolute.

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... And a 'nine day wonder', a 'nine days' wonder' and a 'nine days wonder' are all given as acceptable alternatives in early Google articles. –  Edwin Ashworth Aug 29 at 15:06

I think it all goes back to the -a suffix in Old English that marked plural adjectives, but which has since been lost. Perhaps if we'd had one a thousand years ago, we'd have called it A ten pounda note.

Switching from money to weight, some people would ask the grocer for Ten pound of apples. Presumably that's by association with the form we're talking about here, but I imagine most of us would use the plural in this case. So it's certainly not all cut-and-dried.

In short, it seems this is another case of 'language on the move' in ways we don't normally notice.

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