The Supplementary Vote system is similar to the Second Ballot system used for the French presidential election except that people vote for their first and second choices on the same day. The first choices are counted, the top two candidates remain in the race, and the second preferences of people who voted for eliminated candidates are then distributed between the top two remaining candidates.
Following the recommendations of the Plant Commission, this system was introduced in the UK under Labour for elected mayors and police and crime commissioners. It was favoured as it did not need a change to numbers or preferences but kept to X voting.
The Tories' 2022 Election Act reverted Mayoral elections to FPTP but the Labour government has now returned these elections to SV. Many argue that AV would have been a much better system for electing mayors as SV is designed for a two party system.
The aim of the SV system is to maximise the support for the winning candidate. If no candidate gets over 50% in the first round the winning candidate requires 2nd preferences of those eliminated to win.
However, with more than three candidates in the field, voters face the problem that they don't know for sure who the two candidates in the run-off round will be. Voters who cast both their first and second votes for candidates who are eliminated after the first round will effectively have no vote at all.
As with the Second Ballot system, SV is much better than FPTP for electing a single candidate. However, for parliamentary elections it is not a proportional system.
Where is this system used? UK Mayors (except 2023-26), Norway Mayoral elections and Sri Lanka uses a similar Contingent Vote system
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