Following Gordon Brown's extraordinary, game-changing statement , here is a guide to how a progressive alliance could be constructed. Bear in mind that as Sinn Féin's five MPs refuse to take their Commons seats, a government needs 321 seats for a de facto majority in the House.
Labour: 258 seats
Liberal Democrats: 57 seats
Social Democratic Labour Party: 3 seats (Labour's Northern Irish sister party)
The Alliance Party: 1 seat (Lib Dems' Northern Irish sister party)
Plaid Cymru: 3 seats (currently in coalition with Labour in Wales)
Scottish National Party: 6 seats (the SNP negotiating team  arrived in London last night and called for a "progressive" alternative to a Tory-Lib Dem pact)
Green Party: 1 seat (Caroline Lucas has ruled out  joining a formal coalition, but maintains that she is "interested in talking about ways we might co-operate")
Independent: 1 seat (Sylvia Hermon regularly voted with Labour while an Ulster Unionist MP, and could be expected to back the government on key votes)
Total: 330 seats
Conservative Party: 307 seats (I add one seat, as the Tories are almost certain to win the delayed election in Thirsk and Malton)
Democratic Unionist Party: 8 seats (the DUP generally votes with the Tories and there has been talk of a deal  for some time)
Total: 315 seats
Perhaps the clearest indicator we've had that a progressive alliance is increasingly likely is the statement issued by Nick Clegg this evening. He made it clear he was dissatisfied with the Tories' current offer:
[S]o far we have been unable to agree a comprehensive partnership agreement for a full parliament.
We need a government that lasts, which is why we believe, in the light of the state of talks with the Conservative Party, the only responsible thing to do is to open discussions with the Labour Party to secure a stable partnership agreement.
The possibility that Britain's progressive majority may finally receive adequate representation in government is more real than ever tonight.