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We say

  1. A 300-meter high tower.

  2. A 25-year-old male.

In the first sentence, we didn't hyphenate the noun meter with the adjective high. Yet, in the second sentence, we do hyphenate them. Why is that? Is there a rule to this?

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  • 1
    How do you determine whether a hyphen exists when you say it? – Victor Bazarov Aug 24 '15 at 18:28
  • In these instances, I know the hyphen exists because we I don't pronounce the letter (s) at the end of the words year and meter even though the numbers before them indicate plural. – Ghaith Alrestom Aug 24 '15 at 18:35
  • If you look at these written instances of 300 meter high (and even more for the BrE spelling) I think you'll have to agree that your single hyphen form is relatively unusual (the vast majority have either no hyphens, or two). I've no opinion on "correct" punctuation, but the implication of your initial "We say" shouldn't go unchallenged. – FumbleFingers Aug 24 '15 at 21:45
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The number-year-old is an idiom, it would really make little sense to try to separate the number-year from old:

A 25-year old male.

But a "high tower" is OK alone, and also 300-meter is fine. It is acceptable to hyphenate the dimension and the direction (thus bringing them together), as in "300-meter-high", or to keep them separate -- it does not change the meaning.

It is conceivable that one could try exchanging the positions of the attributes, and say

A high 300-meter tower

While it does sound somewhat unusual, it's still very clear (tower does imply a tall structure, it's not difficult to understand what 300-meter defines). On the other hand, if you see

An old 25-year male.

you cringe because in this case what is a "25-year male"? And, can there be a young 25-year male?

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Hyphenation in American English is not consistent. (Many hyphenated phrases are in the process of being converted from separate words into compound words, and different writers have different opinions about how far any given phrase is in the process.)

The rules I follow are:

  • If recognize the phrase as being a compound word (like "firetruck"), I use the compound word.
  • If the unhyphenated phrase might mean something different from the hyphenated phrase, I use the version that is closer to what I mean.

In the original poster's examples:

  • "300 meter-high towers" has a different meaning from a "300-meter high tower". Suppose I have 300 different piles of blocks, and each pile is one meter high. Those would be "300 meter-high towers". Now suppose I were playing with the blocks while sitting on the roof of a very tall building. That could be a "300-meter high tower".
  • A "300-meter high tower" is a kind of "high tower", so it is consistent with a "300-meter-high tower". Thus, the second hyphen in "300-meter-high" is redundant.
  • Among humans, a "25-year-old male" is not usually considered an "old male". Thus, a "25-year-old male" is different from a "25-year old male".
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  • "300 meter-high towers" has a different meaning from a "300-meter high tower". What is the difference? – Ghaith Alrestom Aug 24 '15 at 18:51
  • @GhaithAlrestom It's the difference between 300 towers each one meter high and one tower 300 meters high. – StoneyB on hiatus Aug 24 '15 at 19:48
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I think OP's question is based on a false premise. It's fine to refer to a 300 meter high[tower] with no hyphens at all, and in fact that seems to be the most common form in those written instances. The second most common form uses two hyphens (which also seems fine to me).

I can't see any grammatical justification for only using one hyphen, and relatively speaking it's not common. I suggest you either hyphenate or don't - but don't "half-hyphenate".

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