Doubling the "L" in Europe and America

Most of the differences between European and American spelling are incidental — not rule bound, or at least not apparently so.

There is, however, a standard way to determine whether or not to double an "l" when adding a suffix.

Europe	United States
When adding a suffix to the end of a word that ends in "l," double the "l." For example, control becomes controlling — and travel becomes traveller.	When adding a suffix to a word that ends in "l," double the "l" only if the final syllable is stressed. For example, "control" becomes "controlling" — but "travel" becomes "traveling."

There's a bit more to it than that, but not much.