I have found many places that list the various rules on using hyphens in math, but nothing to explain why we have the rule. I have some students who are asking and I would like to be able to give them an answer instead of saying it is just the rule.

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    This actually only applies to numbers 21 to 99, at least according to CMS. So five hundred and fifty and nineteen forty-five. Commented Nov 17, 2015 at 21:44
  • @Nathaniel Do CMOS not allow or even recommend 'five hundred fifty'? Commented Nov 17, 2015 at 23:06
  • @EdwinAshworth It doesn't take a strong stance; 9.5 of CMS15 simply says "and may be omitted" in relation to an example. Their website, 5th question, says "Chicago’s preference is to omit it" but again isn't firm. Commented Nov 17, 2015 at 23:13
  • @Nathaniel Cheers. I've always suspected that 'two hundred ten year old trees' (I'm not adding hyphens) may sound ambiguous. Commented Nov 17, 2015 at 23:21
  • "two hundred ten year old trees" is not ambiguous, it means "200 trees that are each 10 years old". "two hundred and ten year old trees" would mean "trees which are 210 years old". Commented Nov 18, 2015 at 0:18

2 Answers 2


To avoid ambiguity.

He turned twenty one too many times.

Is not the same as

He turned twenty-one too many times.

  • 3
    I like your example. I would only add that it's also because the hyphen shows that the number is one unit and should be considered as such (which of course is a corollary of the "avoiding ambiguity" reason) Commented Nov 17, 2015 at 21:56
  • 1
    'In the next field, we have twenty-one-year-old trees' etc. Commented Nov 17, 2015 at 22:55
  • I've heard a joke along these lines before. Q: What's the best thing about dating twenty-two year olds? A: That there's twenty of them! Commented Nov 18, 2015 at 2:24

This is rather similar to TsSkTo's answer, but why not see another usage?

Can I have twenty-five times?

This asks for the multiple 25 times (some unspecified amount or object) or perhaps it asks for twenty-five records of time.

Can I have twenty five times?

This asks for 100.

The hyphen prevents the former number from becoming an adjective describing the latter one.

EDIT: Changed the word cents to times to better match common usage.

  • "five cents" is not a common phrase for denoting a single item, so it's unlikely that this question would be interpreted as $1. One wouldn't say "Can I have a five cents?" but "Can I have a nickel?" or "Can I have five cents?" Commented Nov 17, 2015 at 23:51
  • @user69710 True. I altered the examples.
    – zahbaz
    Commented Nov 18, 2015 at 0:11

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