The Single Transferable Vote (STV) system, part of the whole proportional representation issue, is used in Irish Republic and Northern Ireland Assembly elections but is not part of the British electoral system which at a national level remains based on First-Past-The-Post.
STV is a complicated system.
Constituencies return a set number of 'winners'. Usually this is in the region of 3 to 4.
A party for an election can put forward as many candidates as it likes for a constituency as long as it does not exceed the maximum set i.e. it can't go above 4 for a constituency if this is the figure set.
Voters have as many votes as there are candidates and they place their votes in order of preference.
Voters can vote for candidates from other parties and they can decide their own order of preference.
Seats in a constituency are awarded in proportion to votes cast, with second, third and other preferences of the voters taken into account.
Voters have huge choice under this system. It can favour independent candidates.
STV encourages parties to select a socially varied group of candidates in order to maximise voter preference.
STV favours small parties and independents. Therefore the system produces governments with no overall majorities which can be unstable. STV can create coalition governments with a dominant party dependent on smaller parties for its survival.