second ballot and supplementary vote

The Second Ballot system works similarly to the First Past the Post system, with single-member constituencies. However, if no candidate wins more than 50 per cent in the first round, a run-off is held one or two weeks later between the top two candidates in the first round.

This system is used for presidential elections in several countries (notably in France); it is also used in France for all elections which return a single member.

The SB system is not any more proportional than FPTP and can give rise to problems with tactical voting; it is also expensive to run and can cause "voter fatigue".  While SB it is not a bad system for electing a single candidate (e.g., for presidential elections), the lack of proportionality means it is not the most suitable system for parliamentary elections.

The Supplementary Vote system is similar to the Second Ballot system 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 adopted in the UK for elected mayors and police and crime commissioners.

The aim of the system is to maximise the support for the winning candidate. 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 not a bad voting system for electing a single candidate. However, for parliamentary elections it is not a proportional system.