The Supplementary Vote is a type of voting system. In the UK, it is used in votes for directly elected English mayors like the Mayor of London.
It is a shortened version of the Alternative Vote (AV).
Under the Supplementary Vote, there are two columns on the ballot paper – one for voters to mark their first choice candidate and one in which to mark their second choice. Voters mark one ‘X’ in each column, although they do not have to make a second choice if they don’t want to.
All the first choices are then counted and, if a candidate has a majority, they are elected. If no candidate receives a majority of the votes, 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 added to the overall count is declared the winner.