How many MPs would the Lib Dems gain under PR?
With the Conservatives offering a referendum on the Alternative Vote, rather than full proportional representation, we get to the meat of this negotiating process.
David Cameron seems to be going out on a limb. Risking the wrath of his core supporters in the South, who worry that he's going soft to get into No 10.
But how much has really been given away?
The Electoral Reform Society has this afternoon issued what it says would have been the results of this election, in the South East Government region, under three different systems of voting:
The top line is the number of MPs gained under first past the post. Conservatives won the huge majority of seats.
The next line is the result under STV - the most used form of proportional representation.
LIberal Democrats would add another 19 seats in the South East: MPs like Sandra Gidley in Romsey, David Rendel in Newbury, Evan Harris in Oxford and more than a dozen others could take their places on the green benches.
But on the third line - the Alternative Vote, or AV - there would be hardly any change from the current system.
Dr Ken Ritchie, Chief Executive of the Electoral Reform Society said:
"Gordon Brown's departure has opened the door for coalition talks with Labour.
"Clegg now has three clear choices on voting reform, but only one can deliver on the promise of a New Politics. First-Past-the-Post is the politics of no change, the Alternative Vote, a modest reform for those who don't 'do' change. Only a shift to STV can rebalance our politics in favour of the public.
None of this is a precise science. The chart shows no Green Party MP under STV. The Electoral Reform Society agree that when people knew even small numbers of votes would stack up they would be more likely to go for it.
The model uses the second preference data of a ComRes poll on 26 April 2010. They admit that this data has a number of shortcomings when it comes to estimating how votes for smaller parties might transfer. But they claim that in most seats the simulated outcome is not particularly sensitive to the accuracy of the assumptions made on transfers.
There would also be other variations from the tactical choices we make now, and might make differently under STV. And of course, many others systems are available and already in use in the UK.
Professor Simon Hix of the LSE, Professor Ron Johnston University of Bristol and Professor Iain McLean of the University of Oxford have produced a study of the various trade-offs for the British Academy.
The Conservatives say that AV is their "final offer" - going the extra mile. They are suggesting a referendum at which time all these factors could be properly examined by the British people.
So right now for the Lib Dems it's take it or leave it: Deal or No Deal?
Welcome to the hustings! I'm Peter Henley, the BBC's political reporter in the south of England. From parish councils in Sussex, to European politics in Oxford, this is the blog for you. 
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