I’ve long wondered why there is no apostrophe in words like “writers room,” ‘homeowners association,” or “teachers college.”

Is the noun descriptive or possessive?

I found a good explanation in Emmy J. Favilla’s A World Without “Whom.”

Do not use an apostrophe when a word is primarily descriptive rather than possessive…. The word is acting more like an adjective than a possessive noun.

Grammar Girl agrees with this approach in her post on “Apostrophes.” Here’s how she explains the lack of an apostrophe in farmers market:

The market is used by the farmers, populated by the farmers, but generally not owned by the farmers. So it seems reasonable to conclude that you don’t use an apostrophe because the word farmers is there to identify the type of market. It’s acting like an adjective.

Exception for nouns that don’t end in s

There’s a word for these nouns that act as adjectives. They’re “attributive nouns,” according to Amy Einsohn in The Copyeditor’s Handbook.

And here’s a twist in the use of punctuation with attributive nouns. “If the plural form of the head noun does not end in s, however, the apostrophe is always used: the people’s republic, a children’s hospital,” says Einsohn.

English grammar sure is complicated.

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