Factbox: Hung parliament brings debate on voting system in UK

(Reuters) - Britain's Conservatives and Liberal Democrats said they made progress on Monday at talks to reach a power-sharing deal after an inconclusive election. The Conservatives won the most seats in the election, but fell 20 seats short of a majority in the 650-member parliament.

One possible obstacle to a Conservative/Lib Dem deal could be electoral reform, a long-cherished ambition of the Lib Dems who would win far more seats if Britain switched from its winner-takes-all system to proportional representation.

Below are some details of the possible voting systems for Britain:

* FIRST-PAST-THE-POST:

-- The UK now operates a simple plurality, first-past-the-post system based on 650 constituencies around the country. In this voting system the single winner is the person with the most votes; there is no requirement that the winner gain an absolute majority of votes.

* ALTERNATIVE VOTE:

-- The same constituency boundaries are used and voters would elect one person to represent them in parliament, as now.

-- Rather than marking an 'X' against their preferred candidate, each voter ranks candidates in order of preference. If a candidate receives a majority of first place votes, he or she is elected. However if no single candidate gets more than 50 percent of the vote, the second choices for the candidate at the bottom are redistributed. The process is repeated until one candidate gets an absolute majority.

-- This is used in the Australian House of Representatives.

* ALTERNATIVE VOTE PLUS:

-- AV+ is a mixed system composed of two elements, a constituency element and a top-up. Voters would have two votes - one for a constituency MP and the other from a regional list.

-- The constituency MPs are elected by the Alternative Vote (AV). The so-called 'top-up' MPs are elected on a corrective basis from open party lists.

-- The system is not currently used anywhere but is the system proposed by the Independent Commission on the Voting System to be put to the electorate in a referendum as an alternative to First Past the Post for UK General Elections.

* SINGLE TRANSFERABLE VOTE:

-- Uses multi-seat constituencies and transfers votes that would otherwise be wasted to other eligible candidates.

-- STV initially allocates an elector's vote to his or her most preferred candidate and then, after candidates have been either elected or eliminated, transfers surplus or unused votes according to the voters' stated preferences.

-- This is used in the Australian Senate, the Irish Republic, Tasmania, Malta and Northern Ireland for local elections and elections to the European Parliament.

Sources: Reuters/Electoral Reform Society

(Writing by David Cutler, London Editorial Reference Unit and UK bureau; Editing by Mark Heinrich)