In Old English, long before the Norman Conquest in 1066, many nouns formed plurals without adding S or ES. Instead they added "en." Because some of these are very common words, we still follow the old usage:

Later we began to form more familiar plurals by adding S. And we showed possession of a singular noun by adding 'S, while a plural would be S'—for example, Crawford Kilian'S blog, and the KilianS' house (which belongs to my wife as well as me).

Why don't we say "human" and "humen"? Because "human" comes from the Latin "humanus" (made from humus or earth).

So what to do with these Old English plurals? We treat them as if they were singulars:

By the way, I should add one more point. Marge writes "acapella chorus," but it should be "a capella chorus." "A capella" means "in the style of the chapel" (capella)—where people would sing religious songs without musical accompaniment. Sometimes, to explain English, we have to explain Italian and other languages!

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Hi, I am in education and was wondering if you are alright with student's using this service to improve their grammatical fluency? Would our school need a license agreement, or is the site just open for use by all?

"Hi Jan! Would you like to come with me to Janice? It's a great womens clothing store!"

Or

"Hi Jan! Would you like to come with me to Janice? It's a great women's clothing store!"

Correct me if I'm wrong, however the correct sentence is the first.

In this instance to substitute "women's" for "womens" would be incorrect. The second example implies that the clothing in fact belongs to women (either in part or in whole) which it does not. In this case "womens" takes the form of an adjective. "Womens" is describing what type of clothing the outlet sells, as opposed to a "mens" clothing store.

Again consider the following sentence:

"Those three womens' blouses came from Janice, a womens clothing store."

The blouses belong to the three women, and the clothing store Janice sells womens clothing (that is the type of clothing Janice sells is of the womens variety).

Well, Ben, in the seven years since I posted this item, the rules haven't changed. Possessives of very old plurals like men, women, children, and brethren take 's. They don't take just s, or 's, because they're already plurals. And I can't think of a single word where we show possession by adding s.

The question isn't about how to make men's or women's plural, nor was Ben Pj Rogers's comment. The question is whether or not mens / men's and women's / womens is functioning as a possessive noun, in which case, yes, it would take the apostrophe s, or is functioning as an attributive noun (an adjective) and thus would NOT take the apostrophe s.

Chicago Manual of Style recommends using the possessive form (with the exception of proper nouns such as Diners Club, etc.), by the way (7.25), so women's clothing / men's clothing.