Q. Hi CMOS, I have a (possibly silly) question. Would the phrase “dogs have a tail” be considered grammatically correct? My instinct is that it should be “dogs have tails,” with both nouns plural. On the other hand, there are some contexts where “dogs have a tail” sounds fine, at least to my ear. For example, if someone asked you what the difference is between dogs and frogs, you might say, “for one thing, dogs have a tail.” Is this a quirk of spoken English vs. written English? This is a trivial example, but this issue comes up a lot in the scientific writing I edit. If it’s purely a personal style choice, I’d prefer to stick with the authors’ original wording. Wordings? Thanks!
A. Though some dogs can be silly, your question isn’t silly at all. One problem is that either statement—dogs have a tail and dogs have tails—could be ambiguous if taken literally. The first could mean that dogs as a group share one tail, and the second could mean that dogs each have more than one tail.
But it’s common knowledge that the animal known as a dog normally has only one tail, so you can follow your instincts (which are correct in this case, grammatically speaking) and match plural subject with plural object: dogs have tails. Use a singular object only when the plural might be misunderstood (unlikely with dogs and tails), or when the fact that the object is singular is the point—as in most dogs have only one tail.
This advice applies equally to writing and speaking, but you’re probably right to suggest that a singular object would be more common in speech than in edited prose. For more on this type of construction, known as the distributive possessive, see CMOS 5.25.