I have been told tonight that Tory and Labour leaders feel a currency union with independent Scotland would have to be put to the rest of the UK in a referendum.
Senior Conservative figures say they do not think the party or the country would accept it, therefore feel it is dead in the water as an idea.
Labour's leaders feel the same. They don't want to underwrite Scotland's financial system and they think English voters would agree.
One story dominates Thursday's front pages with some newspapers printing emotive pleas to voters to keep the United Kingdom as one.
In Edinburgh tonight you can hear it and feel it. The buzz, the banter of the day and the thrilling sense something big is coming tomorrow.
There is nervousness, anxiety and on the streets of Edinburgh fuelled by an atmosphere unlike anything seen before in recent UK politics.