Melania Trump in black at the Vatican. Why?

Melania Trump arrived at the Vatican on Wednesday morning the picture of modesty: a knee-length black dress, arms covered and a black veil over her hair.

Her choice of outfit - in particular, the elegant veil - did not go unnoticed by those keeping a close eye on the First Lady during her husband's first overseas tour as president.

Among them was the BBC's North America editor, Jon Sopel.

"Interesting micro point," he tweeted. "Melania Trump wears head covering for meeting with @Pontifex - but not when she was in Saudi Arabia."

But BBC's David Willey in Rome was unsurprised. There is a strict protocol to be followed when meeting the Pope, which the White House will have been told about.

A quick glance at the Vatican website lays out some of the rules: modest dress, with your shoulders covered, for those attending a Papal Audience - especially if indoors.

"It goes from deep lace mantillas to just a black veil," he explained. "When the Queen went to see the Pope when she was younger, she dressed up like the Spanish infanta."

Traditionally, heads of states and their partners have chosen to wear black, with a notable exception - a Catholic queen is allowed to wear white.

Mrs Trump's predecessors have all followed similarly strict rules. Michelle Obama chose to wear a veil on meeting Pope Benedict XVI in 2009, as did Laura Bush and Hillary Clinton on visiting the Vatican during their husbands' presidencies.

There are those who choose to ignore these protocols, of course.

There was an outcry when Cherie Blair, the wife of Britain's then-prime minister, wore white in 2006 - but less so when Camilla, Duchess of Cornwall, wore a pale gold suit, and went without a head covering, in April this year.

"Things have become more relaxed over the last few years there are no hard and fast rules," a Vatican spokesman told The Daily Telegraph newspaper.

Despite this relaxing of the rules, however, Mrs Trump has stuck firmly to tradition. This could be because she is believed to be a Roman Catholic, as suggested by her request for Pope Francis to bless her Rosary beads during the visit.

It should be noted that her stepdaughter Ivanka - who also decided against covering her head in Saudi Arabia - is not Catholic, having converted to Judaism, but still chose to wear a veil while at the Vatican.

Mr Willey was particularly impressed with Mrs Trump's choice of outfit, however.

"To my eyes, what Mrs Trump wore was perfect. She was rather elegantly dressed, and totally in keeping with the event," he said. "The Trump party [as a whole] observed normal protocol and dress."

But why the change from Saudi Arabia? Well, female foreign dignitaries are not required to cover their heads when they visit the kingdom - only Saudi nationals are.

What's more, while the Vatican had spoken of a dress protocol to Mrs Trump's office at the White House, no such requests had been made by Saudi Arabia, her communications director Stephanie Grisham told CNN.