How do you vote in Labour's leadership election?

Labour's electoral system is confusing journalists and voters. Here's how it works. 

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Labour use the alternative vote - a preferential voting system where, instead of putting a "x" in a ballot paper, voters rank candidates in order of preference. 

To win, you need a majority of votes cast - so half the number, plus one. (Our model electorate has 100 voters - so 51.) 

Here's what the ballot paper of our example voter - let's call her Ramsay Blair - looks like:

Ramsay has used all four of her preferences, although she doesn't need to: if she felt very passionately about Kendall, she could just vote Kendall 1 and be done with it. Or if she only wanted a woman, she could give Yvette Cooper her second preference. 

Anyway, on to the voting itself. We have 100 voters, and in the first round, it breaks down like this:

Jeremy Corbyn has 42 votes

Andy Burnham has 25 votes

Yvette Cooper has 21 votes

And Liz Kendall has 12 votes.

No-one has half the votes cast plus one (51 votes). So we go onto the next round. Liz Kendall, who has just 12 votes, is eliminated first. A single tear rolls down Ramsay's cheek. Kendall's votes are now re-allocated based on the second preferences of her 12 votes.

Of Kendall's 12 votes, 8 transfer to Yvette Cooper. Cooper now has 29.

Of Kendall's 12 votes, 1 transfers to Andy Burnham. Burnham now has 26.

Of Kendall's 12 votes, 2 transfer to Jeremy Corbyn. Corbyn now has 44.

Of Kendall's 12 votes, one doesn't rank any of the other candidates at all. We call these "exhausts". 

The electorate is now 99 voters. (Cue tedious "Jeremy Corbyn has 99 problems, but getting 51 per cent of the vote ain't one" gags.) 50 votes are now needed to win. 

Jeremy Corbyn has 44 votes

Yvette Cooper has 29 votes

And Andy Burnham has 26 votes.

44 is smaller than 50, so Burnham goes out. Better luck next time, Andy! Burnham's votes are now re-allocated based on their second preferences. (That Kendall vote that has gone to Burnham's would switch to its third. This is the upside of using all your preferences!) 

Of Burnham's 26 votes, 6 transfer to Jeremy Corbyn. He now has 50 votes. 

Of Burnham's 26 votes, 18 transfer to Yvette Cooper. She now has 47 votes.

Of Burnham's 26 votes, two don't transfer to any of the remaining candidates at all. (If one of them had voted Burnham 1, Kendall 2, Cooper 3, they would still transfer to Cooper, as Kendall has been eliminated. These two only want Andy.)

So we go forward to the final round. The electorate now consists of 97 votes. 49 votes are needed to win.

Jeremy Corbyn has 50 votes.

Yvette Cooper has 47 votes.

Jeremy Corbyn is therefore elected Labour leader with 50 votes. 

See? It's as easy as pie.

Frequently asked questions:

I just want to vote for Jeremy Corbyn. Will my vote still count?
Yes! You can vote for as many or as few candidates as you like. 

Would Liz Kendall dropping out stop Jeremy Corbyn?

It's been suggested that Liz Kendall ought to drop out to help "stop Jeremy Corbyn". But as you see in our example, as she finishes fourth, her votes transfer anyway. Yes, one of her voters didn't want to vote for anyone else, but if Kendall weren't to stand, they wouldn't bother voting anyway. In fact, there's a chance that some of those second preferences for Yvette Cooper would also have stayed home. (And, in both real life and our example, there are second preferences for Corbyn among Kendall's votes too.)

Can you vote tactically under AV?

Sort of.

As we saw, those 6 transfers from Burnham to Corbyn gave him victory. Let's say that all 29 of Yvette's third preference votes would have gone straight to Andy Burnham - that would give him 55 votes, enough to beat Corbyn comfortably.

(This is highly unlikely, but just roll with it, okay?) 

That means that if your main concern is to stop Jeremy Corbyn winning, you are better off voting for Burnham either after Kendall - as she goes out anyway - or just voting for Burnham. 

There is some evidence from CLP nominations that Cooper's preferences are better for Andy Burnham than the other way round. In both YouGov polls, there is a larger number of Corbyn preferences among Burnham voters than there is among Cooper voters. (Although as the most recent one shows Corbyn winning outright in the first round, it's all moot anyway.) 

So now you know. That's how Labour's electoral system works. 

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.