Princess Charlene has been a member of the Monégasque royal family for nearly 15 years now since her July 2011 wedding to Prince Albert. Yet Monday marked her first ever official working visit to one of Monaco’s nearest geographical neighbors: Spain. The Princess joined her husband, Prince Albert, for a two-day trip to Madrid tied to a significant diplomatic milestone between Monaco and Spain.

Spanish Royals Attends The 150Th Anniversary Of The First Diplomatic Mission Between Spain And MonacoCarlos Alvarez//Getty Images

The two royal couples seen arriving at Monday’s event.

The trip intends to highlight 150 years since Monaco established its first official diplomatic mission in Spain on July 14, 1876. To mark the occasion, Albert and Charlene were welcomed by King Felipe and Queen Letizia for an afternoon at Madrid’s Royal Botanical Garden, where the two couples toured a pair of exhibitions mounted by the Monaco Embassy. One traced five centuries of shared cultural and diplomatic history between the two nations. The other highlighted contemporary artistic work from Monaco. Prince Albert and King Felipe also met for a working lunch at the Zarzuela Palace earlier in the day.

Spanish Royals Attends The 150Th Anniversary Of The First Diplomatic Mission Between Spain And MonacoCarlos Alvarez//Getty Images

Queen Letizia and Princess Charlene, wearing an Oscar de la Renta midi lace dress, have a private conversation at the Royal Botanical Garden.

The Royal Botanical Garden itself has a history nearly as long as the international relationship it helped celebrate. Laid out in 1781 under the direction of architects Francesco Sabatini and Juan de Villanueva, it was conceived as a center for botanical science during the reign of King Charles III of Spain, with a particular focus on plant species sourced from Spain’s colonial territories at the time. The garden fell into disrepair during the Spanish War of Independence in 1808 but was restored half a century later. It has operated under the Spanish National Research Council since 1939.

BIO-SINATRA-MONACO-PRINCESS SOPHIE--PRINCE JUAN CARLOS OF SPAIN-//Getty Images

Prince Rainier, Frank Sinatra, Princess Grace, Princess Sofia, and Prince Juan Carlos in 1962. Juan Carlos ascended the Spanish throne in 1975 and abdicated in 2014.

The Wedding Of Crown Prince Felipe & Letizia Ortiz RocasolanoA. Jones/J. Whatling/J. Parker/M. Cuthbert//Getty Images

Princess Caroline at Felipe and Letizia’s wedding in Madrid in 2004.

Monday’s meeting, however, was not the first between the two royal families. Prince Rainier and Princess Grace visited Spain on several occasions for official visits between the 1950s and 1980s, and also attended the 1962 wedding of King Felipe’s parents, Juan Carlos and Sofia. Prince Albert’s sister, Princess Caroline, also represented the Grimaldis at the wedding of Felipe and Letizia when they were still the Prince and Princess of Asturias in 2004.

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Rachel King (she/her) is a news writer at Town & Country. Before joining T&C, she spent nearly a decade as an editor at Fortune. Her work covering travel and lifestyle has appeared in Forbes, Observer, Robb Report, Cruise Critic, and Cool Hunting, among others. Originally from San Francisco, she lives in New York with her wife, their daughter, and a precocious labradoodle. Follow her on Instagram at @rk.passport.