Scottish independence: What might a currency plan B look like?
We know what the Scottish government's Plan A is on the currency for an independent Scotland.
It intends to negotiate to continue to share the pound with the rest of the UK in a formal currency union.
The first minister, Alex Salmond, is convinced that will happen because otherwise, he argues, the rest of the UK would be making itself liable for Scotland's share of accumulated debt.
But the current UK government and the Labour opposition don't accept that analysis and insist there will be no deal.
They keep asking Mr Salmond for a Plan B. So what might that be?
The first minister says he will not compromise his negotiating position by being drawn on this.
Instead, he lists the options. The euro, a separate Scottish currency or using the pound anyway.
But the Scottish government has already said that it does not favour the euro, so that's out.
In the introduction to the White Paper on independence, Mr Salmond says those who would prefer Scotland to switch to "our own currency" would have to win support for that in an election.
That would appear to rule out a separate Scottish currency in the first days of independence, between 24 March 2016 (the Scottish government's target date) and the election in May.
In addition, Mr Salmond has insisted that, come what may, sterling is "our currency and we're keeping it".
So, that leaves using the pound without formal agreement, or sterlingisation as it is known. But for how long?
On BBC Radio Scotland this morning, Mr Salmond repeatedly made clear that this was a suitable "transitional" arrangement, rather than a long term solution. That's based on advice from his fiscal commission.
So, that gets us back to the possibility of a separate Scottish currency, at some point, which is what some key figures in the "Yes" campaign have argued for all along.
Of course, the first minister hopes none of the alternatives will be needed - that a currency union will be agreed if there's a "Yes" vote.
Opinion polling suggests the Scottish public tend to believe that is what would happen. But it's not them that would be negotiating on behalf of the rest of the UK.
Those who favour Scotland continuing in the UK insist that the only way to be sure of keeping the pound is to vote 'No' and keep the currency union we already have as part of our political union with England, Wales and Northern Ireland.
As the people of Scotland weigh up how to vote in the independence referendum, they are asking questions on a range of topics from the economy to welfare.
In a series running up to polling day, BBC correspondents are looking at those major questions and by using statistics, analysis and expert views shining a light on some of the possible answers.