When President Barack Obama stepped into the pope's private library in the Vatican July 10, he became only the 12th U.S. president to do so.
And while the Vatican has a protocol handbook governing visits by heads of state -- a handbook that covers everything, including the number of Swiss Guards and papal gentlemen in tails present -- the way each visit unfolds is determined by the schedules of the pope and his guest.
The fact that Obama came to the Vatican directly from the Group of Eight meeting in L'Aquila, Italy, and left immediately afterward to fly to Ghana meant timing was tight.
The time constraints meant the Vatican and the White House made no plans for an exchange of formal speeches -- an optional part of papal receptions of presidents.
But there is always time for an exchange of gifts.
The Baltimore province of the Redemptorists announced that it had given Obama a stole that had been placed on the remains of St. John Neumann, a 19th-century Redemptorist and the first male naturalized U.S. citizen to become a saint. Obama gave the stole to the pope.
While the gifts presidents and prime ministers give popes are quite varied -- but tend heavily toward old books, statues and vases -- Pope Benedict always gives heads of state a gold medal marking the current year of his pontificate.
The pope gave Obama a medal as well as a mosaic of St. Peter's Basilica and Square and an autographed copy of his recent encyclical. Sometimes -- like when Australian Prime Minister Kevin Rudd met the pope July 9 -- the pope gives his guests a special limited-edition pen shaped like the twisting columns of Gian Lorenzo Bernini's baldacchino over the main altar of St. Peter's Basilica.
It would appear that visiting presidents have a lot more leeway in choosing the gifts they want to give the pope.
In former U.S. President George W. Bush's three Vatican visits to Pope John Paul II, he presented an 1849 first edition of an anthology of American poetry; a silver medallion with a hand-painted image of Mary; and the Presidential Medal of Freedom.
Bush met Pope Benedict at the Vatican twice. In 2007, he gave the pope a walking stick into which the Ten Commandments had been carved by a formerly homeless man. And in 2008, the pope and the president gave each other photographs taken during Pope Benedict's April 2008 visit to the White House.
Some may find it interesting that Obama, who is not Catholic, chose a very Catholic gift for the pope while Japanese Prime Minister Taro Aso, a Catholic, gave the pope a digital video recorder during a similar audience July 7.
The Vatican protocol playbook divides presidential and prime ministerial visits into three categories with an increasing degree of ceremony: private visit, official visit and state visit.
The Obama visit was considered private, but with a bit of flair. He was met in the St. Damasus Courtyard by U.S. Archbishop James Harvey, prefect of the papal household, and a small contingent of Swiss Guards.
In a reversal of the usual order of things, the president met with Cardinal Tarcisio Bertone, Vatican secretary of state, before going to meet the pope.
After a private discussion with Pope Benedict, Obama introduced his wife, Michelle, and daughters, Sasha and Malia, to the pope.
A group photo was taken, then the pope and president exchanged gifts, and the first family left for the airport.
Obama continued a tradition that is only 90 years old.
The first U.S. president to visit the Vatican was Woodrow Wilson, who met with Pope Benedict XV in 1919 while on a European tour after World War I.
The next presidential visit was a full 40 years later; Dwight D. Eisenhower met Pope John XXIII in 1959.
Since then, each U.S. president has made a trip to the Vatican.
Meeting Pope Paul VI were John F. Kennedy in 1963; Lyndon B. Johnson in 1967; Richard M. Nixon in 1969 and 1970; and Gerald R. Ford in 1975.
Jimmy Carter was the next to visit, meeting Pope John Paul in 1980. Ronald Reagan met him in 1982 and 1987, as well as after leaving office. George H.W. Bush met him in 1989 and 1991. Bill Clinton came to the Vatican in 1994.
George W. Bush met Pope John Paul in 2001 at the papal villa in Castel Gandolfo and at the Vatican in 2002 and 2004. In addition to attending Pope John Paul's funeral in 2005, he visited the Vatican in 2007 and again in 2008 to meet with Pope Benedict.
Beginning with Carter's visit, the pope's remarks to his presidential visitors focused on two themes: the need for world peace and the obligation to protect human life, particularly the life of the unborn.
The two topics still top the list of the pope's concerns for the United States.