Throughout the last 500 years the Vatican and the Royal Family have had their share of disagreements. Not least about wives.
But yesterday they were as one. Both about wives, and perhaps more importantly, about the future of the planet.
Protocol would normally dictate that Prince Charles, on his third visit to the Vatican, would initially meet Pope Benedict without the Duchess of Cornwall. But protocol was waived to enable them to meet the Pope together, instead of the Duchess coming in at the end.
And while detail of the conversations were not released – the environment was at the top of the agenda where both men are know to be ahead of their respective countries.
Last week the Pope's sovereign state, which has been aggressively pursuing a low emissions policy for several years, announced that it was investing €500m to equip it with what it boasted would be "the biggest solar panels in the world," to be installed on the roof of the Santa Maria Galleria within the Vatican's grounds. Last autumn a huge set of panels was installed on Paul VI Hall, a modern auditorium next to St Peter's.
And two years ago the Vatican began planting a forest in Hungary on land donated by well-wishers, which is intended to compensate for all the city state's carbon dioxide emissions. "The Vatican wants to play its small part in the elimination of CO2 emissions," commented Paul Poupard, president of the Pontifical Council for Culture, at the time.
"When man forgets that he is the servant of the earth and becomes its master, the earth itself rebels against man and instead of being a place of welcome becomes a desert which threatens the survival of Creation." Sentiments which the Prince would heartily endorse.
Speaking in the Italian parliament, Prince Charles warned that the world risked entering "a new Dark Age" if global warming was not curbed. "Only 99 months remain before we reach the point of no return," he said. Italy, as president of the G8 this year, "was in an ideal position to prove its leadership at a global level."
As the Prince is doubtless aware, however, Mr Berlusconi's Italy has shown little enthusiasm for embracing tough measures to reduce CO2 emissions, and threatened to veto the EU Energy and Climate Package last October.
The Prince and the Duchess of Cornwall enjoyed a tour of the Vatican followed by a 15-minute tête-a-tête with the Pope. The Prince also had a half-hour meeting with the Pope's Secretary of State, Cardinal Tarcisio Bertone.
The last time Charles visited the Vatican was for the funeral of Pope John Paul II in 2005. "He was such a wonderful man, we miss him terribly," the Prince was heard to remark as he was leaving the Hall of the Popes where the meeting with Benedict took place.
Prince Charles gave the pope a set of dessert plates decorated with paintings of flowers from his Highgrove Estate, remarking, "I don't know whether these will be of any use to you," as he handed them over. The Pope gave the couple an etching showing St Peter's before it was re-designed by Michelangelo.
No more was heard of the gift which, according to reports last week, the Pope had been intending to give to the couple: a magnificent facsimile of the letter written by English peers to Pope Clement VII in 1530, pleading for the annulment of Henry VIII's marriage to Catherine of Aragon.
As a key document in the history of the rift with Rome and the English Reformation, it is a fascinating piece of history. But as a present to two royal British divorcees from a Pope whose uncompromising views on the subject are common knowledge, it would have lacked something in diplomatic finesse.