I think we all know that an apostrophe is used before the possessive of singular common nouns, e.g.
the boy’s bike.
So far, so good. But what if the noun being possessed (as it were) isn’t singular, but plural, and ends in S?
Plural nouns ending in S take the apostrophe after the S, e.g.
two boys’ bikes.
However, just to complicate matters, some plural nouns don’t end in S; children is one of them. In that case, they are made possessive by adding apostrophe S, e.g.
the children’s games.
OK. But …
When it comes to the possessive of names, the current ruling is that all names take an apostrophe S, e.g.
Gerald Manley Hopkins’s poetry.
Personally, I think it’s only a matter of time before the possessive of all names ending in S will take the apostrophe only, e.g.
Already, names derived from bible sources, for example, only take the apostrophe, e.g.
At present, though, the situation is fraught, with many different usages for the apostrophe where the personal noun ends in S. One school of thought recommends you use apostrophe S when the personal name is of one syllable, e.g.
And that you use the apostrophe only, if the personal name is of more than one syllable. Thus:
What to do?
If you’re really keen to be on the safe side when submitting to a publisher, enquire about their usage. If not, consistency is the thing. Don’t say Hopkins’ poetry in one place in the manuscript and Hopkins’s poetry in another. As in most things, consistency will get you a long way.
Whatever you do, don’t do this:
But that’s another story.