[duaneb]

I say Mum in the US. I don't think there's a hard or fast rule.

[colordrops]

I don't believe there are any hard and fast rules in the English language. It's all convention, some of which has a very high percentage of adherents, and some people in high towers claiming that one way is more correct than another. But you will find that every "rule" has some form in some region that breaks it. There's the concept of an official grammar and vocabulary in some languages, such as French, with the Académie française, but even then people don't follow the rules. English has no such official committee.