"The British Parliament, after a week or more of wrangling, voted Thursday to hold a referendum on replacing the U.K.'s first-past-the-post voting system with a new method known as the alternative vote, or AV. Under AV, voters will cast not one ballot, but several, in declining order of preference. As votes are tallied, the candidates with the fewest votes will be eliminated, and their supporters' second preferences counted toward the totals of the remaining candidates. This winnowing would go on until one candidate achieves more than 50% of the votes casts. The referendum is the result of a promise made to the Liberal Democrats in exchange for their participation in David Cameron's coalition government."
The WSJ has this report.