Just read somewhere about this puzzling puzzle:

What is the difference between:

  1. I'm doing a twenty four-hour shift tonight.
  2. I'm doing a twenty-four hour shift tonight.
  3. I'm doing a twenty-four-hour shift tonight.

I know number 1 is 20 x 4 hours. Number 2 and 3 is the same as far as I know. Is this true?

  • All of them read the same to me: one shift that is 24 hours long, starting "tonight." The punctuation is varied. – Arm the good guys in America Feb 13 '17 at 0:42
  • You might want to change the example to disinclude definitives and a specified day, because as things are I doubt anybody would really parse "a twenty four-hour shift tonight" as meaning that your shift would take 80 hours. There are not even that many hours in a whole day (including daytime and nighttime), and a problem that is common to all three examples is that there are certainly not 24 hours in a single night. If you don't want to change the example, because you want a definitive answer congruent with the source, I would recommend sharing the source. – Tonepoet Feb 13 '17 at 0:48
  • Perhaps a better example would be a beverage that's sold in both 4-packs and 24-packs. And you're hosting a large party and have calculated that you need about 80 cans/bottles, but the store was out of 24-packs, so you had to buy twenty four-packs. – Dan Feb 13 '17 at 1:42
  • 2
    The first one is wrong in two ways. It is syntactically wrong: "I'm doing a twenty things tonight" is wrong it should be "I'm doing twenty things tonight". Now "I'm doing twenty '4 hour' shifts tonight" is semantically nonsensical because 'tonight' is maybe at longest 24 hours (you know, above the arctic circle). So at most six '4 hour' shifts would fit. – Mitch Feb 13 '17 at 3:03
  • Ok how about we remove "tonight"? – bdbd Feb 14 '17 at 2:45

Compound Numbers that are below one hundred and made up of two words require hyphenation.

Compound adjectives used before a noun are hyphenated as well, requiring "24" and "hour" to be hyphenated in "24-hour shift".

So only example 3, "I am doing a twenty-four-hour shift tonight" is correctly hyphenated. When read aloud, of course, all three orthographies sound the same.

Note that the first example appears to be badly hyphenated rather than referring to twenty shifts of four hours each. Presence of the indefinite article "a" and use of "shift" in the singular indicate only one shift is being talked about.

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A better example of the hyphens altering the meaning would be if you were to isolate the noun phrases in question:

  1. Twenty four-hour shifts
  2. Twenty-four hour shifts
  3. Twenty-four-hour shifts

In these examples each would mean something different:

  1. 20 x 4-hour shifts
  2. 24 x 1-hour shifts
  3. Shifts that last 24 hours
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