Supplementary Voting Explained
Animation and Explanation
The Supplementary Vote (SV) is a shortened version of the Alternative Vote (AV). Under SV, there are two columns on the ballot paper – one for voters to mark their first choice and one in which to mark a second choice. Voters mark one 'X' in each column, although voters are not required to make a second choice if they do not wish to.
All the first choices are then counted, and if a candidate has a majority, they are elected. If no candidate receives a majority, the top two candidates continue to a second round and all other candidates are eliminated. The second-choice votes of everyone whose first choice has been eliminated are then counted.
Any votes for the remaining candidates are then added to their first-round totals. Whichever candidate has the most votes after these second-preferences have been allocated is declared the winner.
See more at: http://www.electoral-reform.org.uk/supplementary-vote#sthash.4tmx14Mh.dpuf