Whose hair is this?
You are correct. It’s Justin Bieber’s hair.
To show that the hair belongs to Justin Bieber, as you can see, we add an ’s to the end of his name.
Whose head is this?
Wrong! It's not Bruce Willis's; it's Britney Spears's head.
To show that the head belongs to Ms. Spears, we also add an ’s. But because her name ends in an s, if we feel like the pronunciation would be too awkward with the extra s, we have the option to simply add the ’ without the s:
Britney Spears’ head
Whose black coats are melding together?
You are correct. Those are the Olsen twins’ coats.
Because the coats belong to both twins, we place the apostrophe after the s, which shows that the coats belong to both twins. If we wrote the Olsen twin’s coats, it would incorrectly imply that the coats only belonged to one of the twins.
To illustrate why apostrophe placement is so important, let’s take a walk down Olsen twin lane.
Remember a few years ago when Mary Kate’s struggle with anorexia was all over the tabloids? Imagine that this was a US Weekly magazine headline:
Olsen Twins’ Struggle with Anorexia Intensifies
The apostrophe’s placement suggests that both Olsen twins struggled with anorexia. Because of the tabloid’s strict policy on the integrity of their material, US Weekly would simply be mortified to discover that their tiny punctuation faux pas incorrectly implied Ashley had an eating disorder too.
Well, that’s it for apostrophes. I’m off to read about where Brad and Jennifer are secretly meeting this week.