What is the Alternative Vote? (AV)
In 2011 the Liberal Democrats (as part of the coalition) called a referendum to introduce AV (Alternative Vote) instead of First Past the Post.
Under AV you rank candidates in order of preference, rather than voting for a single candidate. Candidates are elected outright if they gain more than half of the first preference votes.
If not, the candidate who lost (the one with least first preferences) is eliminated and their votes are redistributed according to the second (or next available) preference marked on the ballot paper. This process continues until one candidate has half of the votes and is elected.
67.9% voted against AV being introduced.
This video from CGP Grey explains exactly how AV works: