So an independent Scotland won't be able to keep the pound, says George Osborne, the latest in a series of hurdles the size of Hadrian's Wall to be placed in front of Scotland's bid to breakaway.
But, like that very wall, that obstacle could crumble, with a little creative thinking.
We at HuffPost UK don't see why Alex Salmond is so keen to keep the pound anyway.
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Fair enough if he doesn't want the Euro, and perish the thought of a new Scots "dollar" (although it is a town in Clackmannanshire).
In fact, when the First Minister sees what we've mocked up as an alternative currency, we're sure he won't be missing the sterling.