St Thomas' Hospital or St Thomas's Hospital?

In the comments to a previous post, reader katypea asks the following:

“St. Thomas’ Hospital” (as per the directional signage around Waterloo) or “St. Thomas’s Hospital” (as per my sheer editorial common sense). Please tell me i won the arguement with The Boyfriend last night…?

It’s an apostrophe question many people struggle with, so I thought my answer warranted a post of its own.

The rule is simple: use 's if you pronounce the second s, but just add the apostrophe on its own if you don't.

For example, when I worked as a writer at UBS I might easily have written a sentence like this:

Huw Jenkins' appointment as the CEO of UBS's investment banking division was announced last night.

So yes, katypea, your editorial good sense was correct. If you fancy launching a campaign to have the signs changed, I'm in. Just watch how you spell “argument”!