For example, in the alphabet, B is "next" after A, but C is "next but one" after A.
I grew up in England but now live in the USA. I had one of those moments today when I said "next but one" and got empty stares and laughs from my colleagues. It still happens to me, even after living here for 13 years!
I would be interested to see whether any Americans know this phrase, and to what extent it's known by Brits and other English-speakers. A Google Books search found some apparently American usages, but they seem quite old.
UPDATE: If you don't know/understand "next but one', would you understand <a href="http://www.amazon.com/All-Laws-but-One-Liberties/dp/0679767320">All the Laws But One</a> to mean "all the laws, except for one"? That was a famous quotation from Abraham Lincoln.