When Princess Charlene of Monaco meets with Pope Francis at the Vatican on Monday, Catholic royal-watchers will be keeping a close eye on her clothes.

What color Charlene chooses to wear during her state visit alongside Prince Albert II next week has major significance as she is one of seven women in the world that has been given “the privilege of the white” – or the ability to wear white when meeting with the pope.

Get push notifications with news, features and more.

Called le privil ge du blanc in French or il privilegio del biacno in Italian, the special tradition is extended solely to designated Catholic queens and princesses and is usually reserved for important events at the Vatican like private audiences, canonizations, beatifications and special masses. According to protocol, other women who meet with the pope are asked to wear black clothes and a matching mantilla, or a lace veil worn over the head. First Lady Michelle Obama followed this rule in 2009 when she joined President Barack Obama in visiting Pope Benedict XVI.

Of course, wearing black (or white) is not required for women who meet with the pope. As head of the Church of England, Queen Elizabeth II has previously worn black and a mantilla when meeting with Pope John Paul II and Pope John XXIII, but she hasn’t followed the protocol in more recent visits.

Currently only seven women hold the privilege of the white – Queen Sof a of Spain, Queen Paola of Belgium, Grand Duchess Maria Teresa of Luxembourg, Queen Mathilde of Belgium, Queen Letizia of Spain, Princess Marina of Naples and, of course, Princess Charlene. The tradition is not extended to non-royal female Catholic heads of state (like former Argentinian president Cristina Fern ndez de Kirchner) or Catholic wives of non-royal heads of state (like former First Lady Jacqueline Kennedy) nor is it extended to Catholic wives of non-Catholic monarchs (like Queen M xima of the Netherlands).

It is also not given to every Catholic monarch’s wife as Princess Marie of Liechtenstein and Queen ‘Masenate Mohato Seeiso of Lesotho apparently do not have the privilege. The women traditionally need to be the queens of a “Most Catholic Majesty” or Rex Catholicissimus – either as consort or in their own right as regnant – a title given to Catholic monarchs by the pope and is considered hereditary unless taken away by the pontiff.

Charlene, who converted to Catholicism ahead of her 2011 wedding to Albert, has exercised the privilege once before when she met with Pope Benedict XVI at the Vatican in January 2013. Her appearance caused a stir amongst royal watchers as the monarchs of Monaco are not traditionally considered to be in the Most Catholic Majesties (hence why the royal families of Lichtenstein and Lesotho are not given the privilege of the white). However, the Vatican’s press office confirmed after the meeting in 2013 that Charlene had been given the privilege of the white – making her the first consort from Monaco to have been included in the special club.

The privilege of the white has even resulted in a minor political snafu. Back in April 2006, Cherie Blair, the wife of former British Prime Minister Tony Blair and an observant Catholic, wore white when she met with Pope John Paul II at the Vatican. Blair’s decision to wear white – even though she wasn’t the wife of a Catholic monarch – raised eyebrows and even drew condemnation from a Catholic British MP who accused her of having “a very grand idea of herself.”

That said, just because these women have the privilege of the white does not mean that they always choose to exercise it. During the inauguration mass for Pope Francis in March 2013, Charlene chose to wear black instead of white. The rules of how to dress are also not set in stone as the ever-chic Letizia decided to forgo the traditional white mantilla when she met with Francis in June 2014. (She followed in the footsteps of her mother-in-law Queen Sof a who ditched the white mantilla when she met with Francis in April 2014.)