It is a form of proportional representation. The idea is that it better reflects the wishes of voters and leads to fewer votes being ‘wasted’.
Under the Single Transferable Vote, voters rank candidates in order of preference. So they put a 1 on the ballot paper next to their first choice candidate, a 2 next to their second choice candidate and so on.
They can rank as many candidates as they like. So if they only want to vote for one candidate, all they have to do is put a 1 next to their name.
In order to be elected, a candidate must reach a set amount of votes known as a quota.
The votes are counted in stages. In the first stage only first preferences are counted. Anyone who reaches the quota is elected. Any votes received over the quota are not needed by the elected candidate and so are transferred to that voter’s second preference.
If not enough candidates have then reached the quota, the candidate with the lowest number of votes is eliminated and the second preference of everyone who voted for that candidate is then added to the tally.
This process is repeated until three or four candidates have been elected.