Recently I got a letter from my phone company informing me that my area code will be switching to 10-digit dialing sometime next year. Several times the letter mentioned that we will have to start dialing “10-digits.” It was very consistent—every time the numeral 10 was followed by the noun “digits,” there was a hyphen between them.
Now, I’ve tried to mellow over the last few years and take a more descriptivist stance on a lot of things, but I’m still pretty prescriptivist when it comes to spelling and style. Hyphens have a few different purposes, one of which is to join compound modifiers, and that purpose was not being served here.
Unfortunately, this is one of those things that most people aren’t really taught in school anymore, and even a lot of editors struggle with hyphens. It seems that some people see hyphens between numerals and whatever words follow them and generalize this to mean that there should always be hyphens after numerals.
But this isn’t the case, because as I said before, hyphens serve a purpose. The stress patterns and intonation of “10 digit(s)” are different in “You have to dial 10 digits” and “You have to dial 10-digit numbers,” because one is a compound and the other is not. The hyphen helps indicate this in writing, and if there’s a hyphen when there doesn’t need to be one, the reader may be primed to expect another word, thinking that “10-digits” is a compound that modifies something, only to find that that’s the end of the phrase.
Of course, one may argue that in compounds like this, the noun is always singular (“10-digit dialing,” not “10-digits dialing”), thus preventing any ambiguity or misreading. While technically true, some readers—like me—may still experience a slight mental hiccup when they realize that it’s not a compound but simply a numeral modifying a noun.
The solution is to learn when hyphens are actually needed. Of course, not all style guides agree on all points, but any decent style guide will at least cover the basics. And if all else fails, trust your ear—if you’re saying it like a compound, use a hyphen. If you’re saying it like two separate words, don’t use one. And if you’re writing or editing anything for publication, you really should know this already.