Apostrophe position for plural possessive

Question

Which of these two are correct?

Ada and Julia are girl's names.

or

Ada and Julia are girls' names.

Answer 1

I would use "girls' names" in this instance. If just one name is mentioned, the apostrophe goes inside:

Ada is a girl's name. Julia is a girl's name.

With more than one name mentioned, the apostrophe would go after the s.

Answer 2

The possession - with how you've written it - doesn't make sense.

Because in this case "girl" is acting more as an adjective, I would write it as:

Ada and Julia are girl's names.

However, if Ada and Julia were the names of specific girls, say:

Ada and Julia are *the* girls' names.

Perhaps less confusing for this specific situation, if you want to avoid apostrophe use at all:

Ada and Julia are girly names.

Answer 3

Consider:

Ada and Julia are names of girls.

for plural possessives in a sentence to avoid this issue altogether.

There is another way around this problem of klunky possessives: using the "of phrase" to show possession. For instance, we would probably say the "constitution of Illinois," as opposed to "Illinois' (or Illinois's ??) constitution." (Source)

Answer 4

Probably because of something to do with the apostrophes, I can't actually reproduce the relevant NGrams usage chart here, but if you follow that link it should be obvious OP's first usage (girl's) effectively doesn't exist by comparison with the "standard" version (girls').

Notice that I included toys as well as names in that first chart. Looking at the same construction using singular forms, it's obvious writers place the possessive apostrophe so as to make the plurality of the "adjectival noun" (girl, boy) agree with the actual noun being modified (name, toy).

Of course, those preferences are purely a matter of orthography, since both versions would be identical in "real" language (speech) anyway. I think it's much more interesting to look at contexts where all native speakers (not just rule-obsessed writers) have to make the choice...

Singular noun...
1: "It's a man's world"
2: "Where's the men's room?"
Plural noun...
3: "He believes that man's interests are asocial"
4: "Do you sell men's gloves?"

Where 2 and 3 are the odd men out (couldn't resist that! :) in that the plurality of the adjective and noun differ. I can only assume 2 is because men's rooms, *men's clubs, and the Wimbledon men's final are so closely associated with multiple men that we depart from general principle. And 3 was the best example I could come up with, because it's so unusual - man there is really mankind, so in the minds of most speakers it's a collective plural noun anyway.