adjectives - Pluralization rule for "five-year-old children", "20 pound note", "10 mile run"


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?



Answer



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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