Everybody is expecting the Unionist parties to state tomorrow that they will never accept a currency union with an independent Scotland.
They're shooting themselves in the foot if they do, because there's good reason to believe that a formal currency union will benefit the rUK more than Scotland because it's good for currencies to be anchored in natural resources (such as oil) and exports (such as whisky) rather than being dependent mainly on volatile financial services.
However, if they really want to cut off their nose to spite their face, there's nothing an independent Scotland can do to save them from themselves, and it's probably prudent to come up with a plan B.
Economists seem to talk mainly about three options: (1) A formal currency union (pound or euro); (2) using another currency without a currency union; and (3) creating a separate currency.
Nobody wants to introduce the euro just now (and even if we wanted to, one of the necessary preconditions is to have a separate currency that can be linked loosely to the euro), and if Westminster vetoes a sterling currency union, that means (1) is out of the picture.
Using the pound informally would be possible, but it's an option that is normally used by rather small countries, and I can't see it being a sensible long-term option for Scotland (although it might be a good idea for a transitional period), so that rules out (2), too.
Creating a separate Scottish currency sounds scary to many voters, but it actually isn't. What's important to understand here is that currencies can be either free-floating or linked to another currency, and those two options are as different as night and day.
A free-floating Scottish currency would indeed be a bit scary, especially at first until Scotland has had time to build up a relationship with the financial markets.
On the other hand, a Scottish currency linked to the pound sterling isn't scary at all. In fact, that's exactly what's already happening at the moment when the Bank of Scotland, the Royal Bank of Scotland and Clydesdale Bank issue their own banknotes. They basically have to store one pound from the Bank of England every time they issue one pound, and that's exactly how a currency board (which is the technical name for a linked currency) would work.
To put it simply, the National Bank of Scotland will put one pound sterling into its vaults (or more likely, into an electronic account) for each Scottish pound it issues. In that way, a Scottish pound is exactly as safe as an rUK pound because the National Bank of Scotland has the means to replace the one with the other if needed.
A currency board would bring many advantages. The person on the street would think of the Scottish pound as a normal pound, just looking different (exactly like today, except that it'd involve coins as well as notes). However, if the rUK economy collapsed at some point, or if the euro suddenly started to look attractive again, it would be easy to break the link and do something else, either floating the currency freely or replacing it with a link to the euro.
Most people will not really care whether we're in a formal currency union or using a Scottish pound linked 1-to-1 to the rUK pound, so if Westminster tomorrow rules out a currency union, it's obvious what the Yes campaign's plan B will be.