I've always had trouble with the proper plural possessive use of the apostrophe. It's just my thing. Others struggle with the difference between "should of" and "should've", but I'm okay with that distinction. Actually, I'm not sure I've ever seen a "should of" that wasn't in error.
Okay, here's a mini-lesson in the use of the apostrophe.Plural words should not use apostrophes.
As in cats, dogs, houses, words, telephones, etc.Possessive words should use apostrophes.
As in Tom's, Matt's, Ken's, Adam's, Kim's, Erica's, etc.When using a conjecture, the apostrophe should replace the letters.
As in couldn't (could not), should've (should have), etc.There are two exceptions to these rules.
1. When the possessive word ends in an "s", the apostrophe should be at the end.
As in Phyllis', Jess', etc.
2. When using the word "it", the rules change.
When using the word as a possessive (it's bone, it's food), there should be an apostrophe.
When using the word as a conjecture (its hot, its over), there should not be an apostrophe.