Errors involving the presence or absence of an apostrophe or the letter s are among the most common spelling mistakes, which is why I mention the problem over on my Services page:
There is some overlap among line editing, copyediting, and proofreading. For example, all include taking care of pesky details like ensuring plural possessives of surnames ending in s are formed correctly.
What does that mean, exactly?
Plurals
The formation of plurals in English looks simple (add an s), until you actually have to do it. Singular apple becomes plural apples. OK so far. But singular baby becomes plural babies. What’s up with that?
The y messes things up. So if there’s a y, you change it to ie, and then add the s.
Words that end in s usually get es at the end to form the plural, as in buses.
Some formations look weird, like to-dos (the things on your to-do list) or how-tos (the instructions for doing things). Some people want to stick an apostrophe in there, but it’s not necessary, even if your kid got all As in school.
Proper names form plurals a bit differently, because you can’t change the spelling of them. If you know two girls named Mary and invite them both for tea, you don’t write the Maries are coming, you write the Marys are coming. If one of them has the last name Chambers, her family is the Chamberses.
Possessives
To show ownership, we usually put an apostrophe-s at the end: the cat’s paw. As long as the noun is singular, this holds, including the baby’s blanket. Sometimes it causes a pileup of Ss, as in The Chicago Manual of Style example a bass’s stripes. That makes people want to remove that s after the apostrophe, but don’t. It belongs there.
Some style books call for removing the s after the apostrophe in proper names that end in s. For example, The Christian Writer’s Manual of Style calls for Jesus’ as the possessive, whereas The Chicago Manual of Style recommends Jesus’s.
If Mary Chambers lives alone and you go to her house for tea, Associated Press style calls for Mary Chambers’ house, but in Chicago style it’s Mary Chambers’s house.
Know your style book and stick to it. Consistency is what matters.
Plural Possessives
Here’s where you start dropping Ss. When a plural that ends in s comes up against an apostrophe-s, we tend to not pronounce that second s, so in spelling, we drop it. A cat’s paw is one thing, but the puppies’ paws are something else. Likewise the babies’ blankets. But the children’s laughter keeps its s because the plural children doesn’t end in s.
If Mary’s parents invite you to their home for tea, you’re at the Chamberses’ house.
The Chicago Manual of Style contains seven pages about plurals and possessives, so this is a sparse summary. But it should be enough to keep you out of apostrophe trouble.