A student recently asked why she had been corrected when she wrote “The teacher’s union voted to strike.” That’s easy: A union of only one teacher would be a lonely place indeed. “Teacher” had to be “teachers.”

But what puzzled the student was why her professor had corrected the error to read “teachers union.” Wasn’t that just as wrong, the student asked, and shouldn’t it be “teachers’ union”?

Not necessarily.

Sometimes, a noun appears before another noun in a way that looks as if it’s a possessive but really isn’t. It’s actually acting as something like an adjective.

The Associated Press Stylebook calls these “descriptive phrases” and says: “Do not add an apostrophe to a word ending in s when it is used primarily in a descriptive sense: citizens band radio, a Cincinnati Reds infielder, a teachers college, a Teamsters request, a writers guide.”

The AP adds: “Memory Aid: The apostrophe usually is not used if for or by rather than of would be appropriate in the longer form: a radio band for citizens, a college for teachers, a guide for writers, a request by the Teamsters. An ’s is required, however, when a term involves a plural word that does not end in s: a children’s hospital, a people’s republic, the Young Men’s Christian Association.”

The Chicago Manual of Style, calls them “attributive forms” and disagrees: “Although terms such as employees’ cafeteria sometimes appear without an apostrophe, Chicago dispenses with the apostrophe only in proper names (often corporate names) that do not use one or where there is clearly no possessive meaning.” CMOS calls for “taxpayers’ associations (or taxpayer associations)” and “consumers’ group (or consumer group).” In that case, the student would be right. It wouldn’t be wrong to add the apostrophe even if you follow AP style, though it might be changed in editing. Call it “fielder’s choice.”

Note that CMOS makes a distinction between singular and plural attributives (yes, we’ve “nouned” that adjective) and calls for an apostrophe only with a plural, or with a proper name.

Speaking of proper names, we’ve all seen those address signs that say “The Smith’s.” So is that family name acting as a descriptive/attributive, or is it an possessive?

Doesn’t matter. But if the apostrophe is used, it’s important to signal that the family within is not full of dunces. There’s (usually) more than one “Smith” in the house, so that family could have its sign read “The Smiths” or “The Smiths’.” If the family were named “Strauss,” the choices would be greater, depending on whose grammar rules the family follows: “The Strauss’,” “The Strauss’s,” The Strausses,” “The Strausses’,” or even the “Strausses’s,” though that last one gets a little excessive.

Here’s another descriptive/attributive quirk. You would say “I’m going to visit the Lee house.” But if you were visiting a specific person in the house, you wouldn’t say “I’m going to visit the Sue house.” You’d say “I’m going to visit Sue’s house.”

Proper nouns used as descriptives/attributives, it seems, are not on a first-name basis with their nouns.

Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years.