It seems that our new ConDem government will be offering us a referendum on changing the electoral system used for general elections from First Past The Post to Alternative Vote (FPTP → AV). The pre-election manifesto status was that the Tories wanted to keep FPTP, the Lib Dems wanted proportional representation (PR), while it was Labour who were the party suggesting a move to AV. A few times the BBC made the mistake of suggesting that AV is a proportional system, but this is not surprising from reporters who are more concerned with personalities than electoral geekery.
There is some self-interest in the parties’ various stances. On the basis of votes cast in this last general election, the Tories would probably lose seats given any of the other systems, Labour would probably gain a few seats under AV, and the Lib Dems would probably gain around a score of seats under AV and over a hundred under PR. Of course, these are hypothetical results, because we can’t be sure how a different system might change the way electors cast their votes (for the data, see this Grauniad article). All the different systems would still have resulted in a hung parliament, but oddly both AV and PR might have made a Lib-Lab coalition more appealing with a stable majority (mainly because the Lib Dems would have more seats). The ConDems offer of AV is a compromise in that the Tories would possibly lose seats but not as many as under PR, and the Lib Dems would gain seats but not as many as under PR.