How does the Supplementary Vote work?

The Supplementary Vote (SV) is a shortened version of the Alternative Vote (AV). Under SV, there are two columns on the ballot paper – one for voters to mark their first choice and one in which to mark a second choice. Voters mark one 'X' in each column, although voters are not required to make a second choice if they do not wish to.

All the first choices are then counted, and if a candidate has a majority, they are elected. If no candidate receives a majority, 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 allocated is declared the winner.

Pros and cons of the Supplementary Vote

The case for	The arguments against
To some extent, SV encourages conciliatory campaigning, as gaining second-preference votes is important.	Unlike the Alternative Vote, SV does not ensure that the winning candidate has the support of at least 50% of the electorate.
It is a relatively simple system to understand.	SV strongly promotes voting for only candidates from the main three parties.
	If there are more than two strong candidates, voters must guess which two will make the final round, and if they guess incorrectly, their second-preference vote will be wasted. In such circumstances it may even be possible for voters to defeat their preferred candidate
	The system can lead to a lot of wasted votes as many of the votes cast in the first round end up not transferring and being counted in the second round
	SV does not eliminate the likelihood of tactical voting.