I searched but couldn't find the rule. Is a hyphen required for a compound modifier used with mph? I understand it is used for something like "a 6-percent increase."


(1) A 20-mph threshold. The threshold will....

I will use it as shown above in a research report, before explaining further.

  • 1
    I see 20mph limits (simplest), 20 mph limits, and 20-mph limits. Jul 20 at 15:39
  • 1
    The vast majority of internet examples of << 30 mph limit >> seem hyphenless and spaced as shown. Jul 20 at 18:10

1 Answer 1


Any discussion of hyphenation should bear in my what Sidney Greenbaum a committed descriptivist, it should be said) has to say in his Oxford English Grammar (11.33 - Hyphens in Compounds).

The main function of the hyphen is to link words that form a compound word. Compounds may be 'open', written in separate words (eg washing machine), 'hyphenated', linked by a hyphen (e.g. tax-free) or 'solid', written in one word (e.g. handkerchief). Also to be considered are hyphens that attach some prefixes to an existing word to form a new word (e.g. ex-husband). American English tends to use fewer hyphens than British English, but British English practice is increasingly following American practice in this respect.

Notwithstanding this caution he is clear about number compounds. He says simply that

Number compounds are hyphenated (11.33.6)

He gives, among the examples of this,

A 70-mile-an-hour speed limit.

On this parallel, A 20-mph threshold is appropriately hyphenated.

However, as Edwin Ashworth explained in his comment, above, the online evidence shows that the unhyphenated 20 mph threshold is now much commoner than the hyphenated version. This does not necessarily entail that the hyphenated version has become incorrect. It is becoming (or has become) non-standard.

  • 2
    How old is this advice? The vast majority of internet examples of << 30 mph limit >> seem hyphenless and spaced as shown. Jul 20 at 18:12
  • They always omit that the real need for hyphens is to reduce ambiguity, and this is why it seems like the wild west when looking for examples. "Tax free" is fine, but "tax free food" might be better served as "tax-free food" because it avoids the interpretation that there is a tax on food that is free.
    – Yorik
    Jul 20 at 18:50

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