Supplementary Vote

The Supplementary Vote electoral system allows voters to cast a first and second preference vote on one ballot. If no candidate gains at least half of all first preference votes, then the two leading candidates go into a run-off round and all other candidates are eliminated. The second preference votes on the eliminated candidates' ballots are then counted and added to the leading candidates' totals. This is designed to ensure that the elected candidate has closer to majority support than would be the case under First-past-the-post. Voters do not need to cast a second preference vote, and cannot vote for the same candidate twice. SV is used for Police and Crime Commissioner elections in England and Wales, and for the positions of directly-elected mayors in England, including London, Manchester, and Liverpool.

Example election

Taking the results of the 2016 Police and Crime Commissioner election in Nottinghamshire, the Labour candidate won less than 50% of the first preference vote, so a second round was needed. The second preference votes from the eliminated independent and UKIP candidates were counted, and were awarded to either Labour or Conservatives.

As you can see, of the 41,773 votes cast for the three eliminated candidates, only 16,773 cast second preference votes for either Labour of Conservative. Some voters may have decided not to cast a second preference vote, or may have voted for one of the other eliminated candidates as a second preference.