Although there are still seven months to go before Scotland holds its referendum on independence, the three main political parties at Westminster have decided to play hardball. George Osborne, Ed Balls and Danny Alexander have put aside their differences and said that if Scotland votes yes, London will refuse to take part in the currency union sought by Alex Salmond.
The warning that an independent Scotland could not keep the pound is an attempt by the pro-union camp to suggest economic policy would be left rudderless in the event that Salmond and his SNP colleagues win September's poll. There are, though, a number of options available to the yes camp.
Option one is to do nothing. Nicola Sturgeon, Scotland's deputy first minister, says the joint pledge by Osborne, Balls and Alexander amounts to bullying on the part of a campaign panicked by the narrowing of the no camp's poll lead. The SNP could assume that London is bluffing, aware that the rest of the UK would suffer significant collateral damage in the event of a messy divorce.
Option two is to assume that the three main Westminster parties mean what they say and quietly agree to accept whatever terms the Treasury and the Bank of England demand. There will be a high-stakes poker game: while London does not hold all the cards, it has most of the good ones.
Option three would involve Scotland continuing to use the pound even if London said it couldn't. The free-market Adam Smith Institute says it is feasible and sensible. The institute's research director Sam Bowman said Panama, Ecuador and El Salvador all used the US dollar without permission, so there was no reason Scotland could not use the pound without London's permission.
"An independent Scotland that used the pound without the English government's permission, with banks continuing to issue notes privately, would probably have a more stable financial system and economy than England itself".
The fourth option would be to return to the one that Salmond appeared to favour before the financial crisis, namely joining the euro. But emulating Ireland as the new Celtic tiger inside monetary union does not look as attractive as it did before the Irish banks went belly-up. Moreover, negotiations with the European Central Bank and the European Union would be difficult given that a monetary union exists between Scotland and the rest of the UK.
Finally, there's the spurned option of going it alone: a Scottish central bank issuing a Scottish currency and setting its own interest rates. But the new government would need to win the confidence of markets. Doing that would mean higher interest rates than in the rest of the UK and spending controls.