I’m sure this is a duplicate question, but here’s something I wrote for a friend. If your noun is singular and ends in s then whether you add another s after the apostrophe is a matter of style, not grammar.
The London Underground has a station called St. James’s Park (after the Royal Park of that name). There is a stadium in Newcastle called St. James’ Park, which is pronounced like the Underground station.
Apostrophes and how not to be confused
Apostrophes are easy. Here’s a short summary which your teachers could have used. There isn’t even a test at the end. Note: I’m criticising the education you were given, not you for having suffered that.
Simple plurals never have an apostrophe, even if you’re a greengrocer.
- My lists are scattered all over the house.
- All the tomatoes are green.
- I was born in the 1960s.
- All MPs are [fill in something here].
Possessives do have an apostrophe. Write the noun, with its plural “s” if it’s a plural and then put an apostrophe, and add an s if necessary. You don’t need another s if you’ve already got one.
- The car’s owner ran off. (One car, then the apostrophe, then an s because you don’t already have one)
- My MP’s expenses are entirely above board. (One MP, apostrophe and s)
- The cats’ owner fed them. (More than one cat, then the apostrophe and you don’t need another s)
- The soldiers’ CO was awarded the DSM. (More than one soldier, apostrophe and no additional s)
- James’ book was blue. (One James, then the apostrophe, and you don’t need another s although you could add one because James is singular)
- The sheep’s wool was white. (One sheep, apostrophe, s)
- The sheep’s wool was white. (Two sheep, apostrophe, s)
Possessive pronouns don’t have an apostrophe.
- Their car
- The car is theirs.
- Its wheels are chrome.
- Your car is black.
- The black car is yours.
“It’s” only has an apostrophe when it’s a contraction of “it is”, and the apostrophe indicates a letter missed out, in much the same way as “don’t” for “do not” or “you’re” for “you are”. When it is a pronoun you want to make a possessive pronoun, remember the rule that possessive pronouns don’t have an apostrophe.
- It’s a car. Its wheels are chrome.
One reason given for why possessives have an apostrophe is that they are actually contractions similar to “don’t/do not”. For example, there is a 16th century dance called “Lord Salsbury his Pavan”, which became shortened to “Lord Salsbury’s Pavan” because it’s easier to say. That’s why possessive pronouns like “theirs” and “your” have no apostrophe: they’re not contractions. This may or may not be correct — evolution of language is more complex than that — but it’s a handy way of remembering.