The difference is in the number of authors that we are talking about. If there is only one you’d use “author’s”. “The author’s books.”

If you are talking about more than one author we have to use the plural, adding an “s”. “There are two authors in the room.” The apostrophe comes in to play if something belongs to them. This time though you add it after the final “s”. “Those are the authors’ books.”

The shift from “author’s books” to “authors’ books” tells us that there’s more than one author in the second example.

You only use this form when the word adds an “s” in the plural. If it doesn’t (like women in the original example) you form the possessive with apostrophe then “s”, so “women’s toilet”.

I hope this helps. Let me know if you’ve any more questions.

Joanna

I can’t really imagine the signwriter thought the word for “more than one female” was “womens” so yes in some way it’s a possessive problem. They were thinking about the women’s toilet but forgot the apostrophe. Of course they could have saved themselves some bother by avoiding the possessive altogether though which I think is what happens on most signs: “Women”. “Men”. Nice and easy.

There is a different question as to how much this really matters – yes it looks bad on the writer, and it is a good reminder that apostrophes matter – but at the end of the day does it still work as a clear and effective sign?

Joanna