ROME, Jan 13 (Reuters) - The Italian parliament will convene on Jan. 24 to begin voting for a new head of state to replace the outgoing Sergio Mattarella. The voting procedure is complex, with ballots cast in secret, often making it difficult to predict the result. Here are details on how the vote takes place. WHO VOTES? Some 1,008 people should be eligible to take part in the election: the 630 members of the lower house of parliament, the 320 members of the upper house Senate, including five life senators, and 58 delegates dispatched by regional governments. However, the number may rise to 1,009 if a vacant parliamentary seat is filled this month. HOW DOES THE VOTE PROCEED? Four covered voting booths are set up in the lower house of parliament and voters enter one-by-one to cast secret ballots. This makes it hard for party leaders to impose their will. Each vote typically lasts more than four hours. To get elected, someone must win two-thirds of the vote in any of the first three ballots. Thereafter, a simple majority is required. HOW LONG DOES IT TAKE TO ELECT A PRESIDENT? Only three presidents have been elected in the very first round, the last one being Carlo Azeglio Ciampi in 1995. Nobody has ever been elected at a second or third ballot. Things tend to get serious at the fourth ballot. Four presidents have been elected at this stage, including the latest incumbent Sergio Mattarella. The longest election was that of Giovanni Leone in 1971, who became head of state at the 23rd ballot in a vote that was spread out over 16 days. WHO CAN BE PRESIDENT? Under article 84 of the Constitution, the only prerequisite for potential presidents is that they hold Italian citizenship, are at least 50 years old, are registered to vote and are not legally barred from holding office. There are no official candidates and you do not have to be a politician to get the top job. Three of the last four presidents were independents. There has never been a female head of state in Italy. HOW LONG DOES THE PRESIDENTIAL TERM LAST? Seven years. Only one man, Giorgio Napolitano, has been re-elected. That happened in 2013 to overcome a stalemate, and he stood down two years later when the political climate improved.