The supplementary voting system is used for Mayoral and Police and Crime Commissioner elections.
There are two stages to the supplementary vote process:
The first preference votes are counted.
If a candidate receives more than 50% of these first preference votes then he or she is elected and a result is declared.
If no candidate receives more than 50% of the first preference votes then the two candidates with the highest number of votes go forward to stage two.
The second preference votes are then checked for the eliminated candidate(s). Any second preference votes for the remaining two candidates are added to the votes they received in stage one.
The candidate with the highest number of votes received after adding together the results from stages one and two is elected and a result is declared.
See Supplementary Vote (SV) on the Electoral Reform Society's website for more information.