supplementary-vote system

The supplementary-vote system is a method of election where voters rank their top two preferences. If no candidate secures a majority of first-preference votes, only ballots indicating the top two vote-getters as a first or second choice are considered. The winner is then determined by combining these first and second preferences. This system is typically used in single-member constituencies and was notably instituted for London mayoral elections.1

It is a variant of majority voting systems, which aim for a winner to receive more than 50 percent of the vote. Unlike plurality systems where a candidate only needs more votes than any single opponent, majority systems require a broader consensus. The supplementary-vote system is designed to address the challenge of achieving a majority when there are more than two candidates.1