For leaking secret papers the trial of Pope Benedict XVI's former butler and a Vatican computer technician has no precedent since the Holy See became a sovereign state in 1929.
The 19th-century courtroom where the trial takes place is tucked away behind the apse of St. Peter's basilica in a corner of the city state that is strictly off-limits to the millions of tourists who visit the Vatican every year.
The Vatican's criminal court is separate from the legal system of the Catholic Church but the pope holds sweeping powers, including the right to pardon suspects before or during the trial, as well as after a conviction.
"The Holy Father holds legislative, executive and judiciary powers. Court rulings are issued in the name of the Holy Father," Giovanni Giacobbe, a law professor and appeals court judge at the Vatican tribunal, told reporters.
The Vatican court holds only around 30 trials a year -- almost all of them involving petty theft at the expense of tourists or small drug offences.
Bigger cases -- such as the attempted assassination of late pope John Paul II in St. Peter's Square in 1981 -- are usually transferred to Italian courts.
The tiny courtroom where the trial will be held has a crucifix at the centre above the three lay Italian judges who will try the case -- presiding judge Giuseppe Dalla Torre along with Paolo Papanti Pelletier and Venerando Marano.
Prosecutor Nicola Picardi -- known as "promoter of justice" -- is also lay.
A portrait of Pius XI, who concluded the Lateran Accords of 1929 with the Kingdom of Italy, hangs over the wood-panelled judge's chambers.
Among the few concessions to modernity in the 50-seat court are a brand new metal detector outside and the plastic chairs in the gallery.
The trial is theoretically open to the public but this is only possible with special permission from the presiding judge, so in practice few if any people not involved with the court proceedings are expected to attend.
The three judges have Italian citizenship, along with computer technician Claudio Sciarpelletti, while the former butler, Paolo Gabriele, is one of just 594 citizens of the Vatican -- the world's smallest sovereign state.
Gabriele is accused of aggravated theft for allegedly stealing confidential Vatican papers, a charge that carries a sentence of up to four years. Sciarpelletti faces up to a year in prison for abetting the crime.
"This is a unique trial, it has no precedent," Carlo Cardia, a professor of ecclesiastical law at Rome III University told La Repubblica daily.
The last comparable trial was in 1971 when four employees of the Vatican's telephone exchange were tried for stealing papal medals from then pope Paul VI's chambers while he was away, a trial that lasted six days.
Two of the four defendants were found guilty and sentenced to three years in prison but were pardoned by the pope and never served time.
An investigation into a Swiss woman who knocked down the pope at Christmas Mass in 2009 never led to a trial since she was found to be insane.
The Vatican has no prison and if sentenced Gabriele and Sciarpelletti would have to be transferred to an Italian prison under a pre-existing agreement.
Even if they were sentenced, Gabriele and Sciarpelletti would probably not actually do prison time until the result of a possible appeal was clear.
After his arrest in May, Gabriele was held for 53 days in one of three security rooms at the headquarters Vatican gendarmerie -- a sort of police force with around 130 members that is separate from the Swiss Guards.
Gabriele at the time was only allowed out to attend mass but has since been put under conditions of house arrest. He is not obliged to attend the trial although it is expected he will in order to justify himself.
Witnesses including high-placed prelates could also be called to testify during the trial. The pope's secretary, Georg Gaenswein, has spoken to investigators and given evidence against Gabriele and could be called again.
A key question is whether the ongoing investigation could lead to a new trial. Many reports in the Italian press suggest that more people were involved in the plot to leak. Gabriele himself has said there could be "around 20".
The criminal code the trial is taking place under dates back to the late 19th century after the Vatican lost its independence following the Unification of Italy in 1870. It was updated in 1913 and 2008.