FACTBOX-Italy's messy electoral system

ROME (Reuters) - Italians blame electoral laws for chronic instability that claimed Italy's 61st government since World War Two on Thursday with the resignation of Prime Minister Romano Prodi.

President Giorgio Napolitano may decide to install an interim government to revise electoral system before calling Italians to new elections.

Here is a summary of the main points in the debate:

PR VS PLURALITY

For most of its post-war history Italy has used proportional representation, which in its purest form matches the percentage of votes won with a proportion of seats in an assembly. PR can be "tempered" by averages and thresholds to produce workable majorities and limit the number of parties with seats.

The alternative is a "plurality" or majority system, under which the party with the most votes wins.

From 1993 to 2005 Italy tried a mixed system giving three quarters of seats by majority and the rest by PR. This created the current "bipolar" system of broad coalitions.

NEW RELIGION

In 2005 the conservative premier Silvio Berlusconi, who had once said plurality was his "religion", pushed through reforms to damage the chances of challenger Prodi in 2006 elections.

Prodi called it an "attempted coup" and the reform was even dubbed "porcata" (rubbish) by its right-wing author.

It was more proportional, with a threshold of 2 percent for parties in a coalition and 4 percent for single parties, which permitted about 24 individual parties to take seats in 2006.

It awarded "bonus" seats to the winners, allocated on a national basis in the lower house and regionally in the Senate. So when Prodi won in 2006, he had a comfortable majority in the lower house but a margin of just two seats in the Senate. His Senate majority vanished when a small party of Catholic centrists abandoned him.

SPANISH OR GERMAN?

Debate centres on whether to imitate Spain's tempered PR, Germany's less modified PR or a mix. The German system favours wide coalitions rather than single-party governments while a 5 percent cut-off excludes the smallest parties from parliament.

Spanish PR has "corrective devices" which have produced no coalition governments since democracy returned in 1977. It allocates seats by the D'Hondt method which rewards parties with the highest average. Spain has a 3 percent cut-off.

Berlusconi and his centre left rival, Rome Mayor Walter Veltroni, who runs the Democratic Party (PD), had held discussions about a formula that would blend the two, though the PD opposes bonus seats for the winner.