The word conclave (from the Latin clavis or key) was first used in 1271 at the election of Blessed Gregory X at Viterbo, north of Rome. This followed the death in 1268 of the French Pope Clement IV, a widower with two daughters who became a Carthusian priest after his wife died. The longest and most bizarre of all conclaves, it lasted nearly three years.

After the cardinals had argued for 18 months, causing increasing anger and rioting in the town, on Whit Sunday 1270, its civil rulers, on St Bonaventure's advice, ordered soldiers under a Captain Gatto to surround Viterbo's papal palace, remove its roof, wall up its doors and put the cardinals on a diet of bread and water until they elected a pope. They eventually chose Gregory, who was not yet a priest and was then in Syria. It took him six months to return home, be ordained and consecrated pontiff. So the Church was without a pope for three years and four months, the longest time in its history.

To prevent harm to the Church by such long conclaves, in 1274 a General Council at Lyons in France made new rules, one being that the cardinal electors should all sleep in a dormitory. The next conclave, the first that can really be so called, was held in 1276 at Arezzo, near Florence, and on the first ballot elected the Frenchman, Blessed Innocent V. Yet 13 years later there was another long period without a pope 11 months, during which six of the l6 cardinals present died from the plague - until the election of Nicholas IV.

Early papal elections were often marred by skulduggery and violence. After St Damasus was elected 366, his supporters stormed another church in Rome where a rival for the papacy had installed himself. In a three-day battle 137 people were killed.

A century after the election of St Symmachus, a Roman synod in 499 strictly forbade would-be popes to canvass for themselves. It also tried "to stop the intrigues of bishops and riots among the people". This is the first known decree regarding papal elections, but conflicts and corruption surrounding them continued for many centuries. Indeed soon after the 499 Roman synod, St Felix IV was the only pope to appoint his successor, Boniface II, who took over from him the day Felix died in 530.

In 686 the army occupied a Roman basilica and "elected" a general's son, Pope Conan. And in 731 during the funeral of St Gregory II, a Roman crowd "elected" St Gregory III by acclamation. A few others became pope the same way.

In the tenth century a Prince Alberic, who ruled Rome, appointed four successive popes. A century later John Crescentius, head of a local "mafia" family that controlled the city, made three of his relatives popes one after another. And a few years later the German Emperor Henry III appointed four German popes.

In 1241 Rome's civil ruler imprisoned ten cardinals in a ruined palace with the body of Gregory IX, who had died the previous day, until they elected his successor. The weather was so hot that one of them, an Englishman, died during the 70 days it took them to choose Celestine IV, author of a history of Scotland, who also died two weeks later. The remaining eight cardinals then spent 18 more months at Anagni, near Rome, before they elected Innocent IV, an unscrupulous nepotist.

The next 500 years saw many more long conclaves. There was one of 11 months in 1288, during which 16 cardinals died; one of 72 days in 1550, when the English Cardinal Pole, a cousin of King Henry VIII, missed being elected pope by one vote; two of 80 days each in 1655 and 1670; a six-month one in 1740, during which four cardinals died; a three-month one in 1769, when it took 185 ballots to elect the Franciscan Clement XIV, who suppressed the Jesuits. The last long conclave, over four months in 1775, elected Pius VI, who reigned for 24 years.

In contrast to these, there were four relatively short conclaves during 18 months in 1590 and 1591, all dominated by King Philip II of Spain. The shortest conclaves, lasting less than one day, were in 1503, when Julius II, who had three daughters, was elected; the election in 1572 of Gregory XIII, who tried to have Queen Elizabeth I assassinated; and in 1939, when Pius XII was elected.

Conclaves were held in some 20 places besides Rome. But since the election of Martin V at Konstanz in Germany in 1417, only one conclave was not held in Rome. This was the one - its expenses were paid by the last Holy Roman Emperor, Francis II - which elected the Benedictine Pope Pius VII in his order's monastery at Venice in 1800.  All the popes since then have been elected in the Vatican.  But before the conclave there in 1878, which elected Leo XIII, he and most of the cardinals thought it unsafe to hold it in Rome, owing to civil strife at the time, and some wanted to have it in Spain.

For more than 1,000 years, the popes, being bishops of Rome, were elected, at least in theory, by the clergy and laity of Rome, as were the bishops of most other dioceses. But in 1059, during a synod at the Lateran, Pope Nicholas II issued a decree restricting elections to cardinals. And in 1622 Gregory XV, when reorganising conclave procedures, introduced the secret written ballot. His reforms have remained largely unchanged, although it took centuries to eliminate outside political pressure.

The last attempt at this was during the conclave in 1903 after the death of Leo XIII. Cardinal Puzyna, Archbishop of Krakow (then in the Austro-Hungarian Empire), acting on instructions from the Emperor Franz Joseph, tried to veto the election of the leading candidate, Cardinal Rampolla, Pope Leo's Secretary of State. Though the other cardinals strongly protested at this, Rampolla lost out to St Pius X, who was chosen instead. One of his first decrees was to prohibit the veto, which was often used at conclaves by powerful Catholic rulers.

The smallest number of electors at a conclave was in 1277, when seven cardinals elected Nicholas III, the first pope to live in the Vatican. And the greatest number, 115 from some 50 countries, went to the conclave six weeks ago that elected Pope Benedict XVI.