Or, to be more precise, sterling and the UK national debt in the case of the independence debate.
The currency question is becoming one of the most important issues of the referendum as so much else depends on it.
It is not about whether we have William Shakespeare or Robert Burns on our fivers and national identity in the way the pound versus euro debate has been characterised by some in England.
There are implications for interest rates and therefore mortgages and pensions, as well as freedom to pursue the more socially progressive policies that have been promised.
One of the most attractive reasons for independence to many is the ability for Scotland to reject Westminster austerity and privatisation and allow better resourced public services and adequate welfare provision.
This would be at odds with Westminster as whatever government is in power next year will continue with cuts leading to divergent political directions of the partners in the currency union.
The compromise might be Scotland has to rein in spending plans, which would be counter to the independence aims and we end up where we are now - blaming London for the inability to act as we want.
The Treasury advice to the Chancellor stated "relations between the nations of these islands would deteriorate, putting intolerable pressure on the currency union".
Political bluff or not, it has been stated and the UK blocking a currency union now must be considered a likely outcome.
A CURRENCY union with another country obviously requires the consent of the other, so there must be an alternative plan.
After witnessing the crisis in the Eurozone and the banking crash here that led to the austerity regime, people need confidence in our economic system and are right to be concerned.
The alternatives must be explored now rather than waiting till after and if Scotland votes yes, leaving the Scottish Government going into negotiations with just one position.
The alternatives are the euro, which involves the same ceding of some autonomy and swapping the Bank of England for the European Central Bank.
Then there is the option of a new Scottish currency, like those nations to the north, such as Norway that we hear much of in terms of public services.
If independence is dependent on sharing the pound the UK may have dealt the yes campaign a fatal blow unless alternatives are fully explored - sooner rather than later.