NOTE: This article is actually an e-mail comment that Thomas Scott wrote in response to my article "Hyphenation: Part III (Suspensive Hyphens)." In his comment, he so effectively covered subtle issues of usage that I had not dealt with in the original article that I asked his permission to include his comment as an article on this website.~Tina Blue
As a copy editor, I get into all kinds of trouble with my interpretation of suspensive hyphenation, as I'm sure many of us do. Your explanation serves most instances when this is used, and I think you're right in warning writers not to overdo it.
The one area where suspensive hyphenation is used at my newspaper makes no sense to me. When describing the dimensions of a rug, for instance, some seem to think it would be "the 9- by 12-foot rug." But there's no range of anything being described; it's just a single, unchanging rug. It's the range of something that's the key to using suspensive hyphenation, in my mind. The example tells me there's a 9-foot rug by 12-foot rug, which isn't what is meant. Because it's a single rug measuring 9 feet by 12 feet, it follows that it should be "the 9-by-12-foot rug." What's more, in a simple sentence describing the rug, it would be, "The rug is 9 by 12 feet." By moving the dimensions in front of the noun, they become a single modifier that should be hyphenated together.
One way to see how "by" uses don't quite qualify for suspensive hyphenation is to talk about rugs with different dimensions. So we have this example: "All 9-by-12- and 10-by-14-foot rugs are on sale." That means hurry up and get your 9-by-12-foot rugs and 10-by-14-foot rugs while the sale lasts. How would a person decipher "the 9- by 12- and 10- by 14-foot rugs"? The 9-foot rugs by 12-foot rugs and 10-foot rugs by 14-foot rugs? Huh?
The AP Stylebook wants to back me up on this one in its dimensions entry. Unfortunately, it doesn't go far enough. It gives the example "the 9-by-12 rug" (that's where I came up with that!) but fails to include a dimension.
Some apparently think adding the dimension blows the single modifier into a suspensive hyphenation construction, but why should it? Some take my argument further and demand that a fully hyphenated single modifier should be used in instances using "to." Example: "The 10-to-20-year prison sentence . . . " And I can buy into that because we are talking about one sentence, but within that sentence is a range of years. "The prison sentence was 10 to 20 years" bears that out. So the sentence actually ranges from 10 years to 20 years. The convict could wind up serving 12, 15 or 18 years, all of which are different sentences in terms of length of time. So I stick with "the 10- to 20-year sentence" and hope I'll never have to serve one.