Usage and Grammar

Q. Hello CMOS. Per your style, for phrasal adjectives including units of measurement, I’ve used the singular form of the unit in, for example, “100-foot-long boat” (instead of “100-feet-long boat”). An author has rejected my edits that revise “feet” to “foot”; telling him that this is incorrect because of a style guide has not convinced him to revert them back to the singular. Why, exactly, should the singular be used? I’m accustomed to it, but I’m unable to come up with a compelling reason.

A. Though it can depend as much on personal idiom as on logic, an argument in favor of “foot” rather than “feet” would be based on the fact that nouns used attributively are usually singular rather than plural. For example, a doctor who treats feet would be called a foot doctor, not a feet doctor. Or, to use an example that’s analogous to yours, plumbing that’s 100 years old would be 100-year-old plumbing, not 100-years-old plumbing. By the same token, you would refer to a 100-foot-long boat or, if the dimension goes without saying, a 100-foot boat. Whatever you do, stay dry.