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Main differences in the voting systems explained

The Labour leader Ed Miliband is joining forces with senior Lib Dems in launching the Yes vote in the referendum on changing the system for electing MPs.

But polls show that most people do not know what the alternative vote is.

So what are the differences between the current first-past-the-post system and the proposed AV method?

Nick Robinson has been to Plumpton races to find out.