Key members of Thailand's royal family -- King Bhumibol Adulyadej, from left, Crown Prince Maha Vajiralongkorn and Queen Sirikit -- pose for a picture in 1999.

Hamad bin Isa Al Khalifa, shown here in November, is King of Bahrain.

Bhutan's King Jigme Khesar Namgyel Wangchuck, shown here with his wife, Queen Jetsun Pema, is known as the "Dragon King." He became king of the South Asian country after his father abdicated in 2006.

Britain's Prince Philip, Duke of Edinburgh, and Queen Elizabeth II have been married since 1947. Her coronation was held in June 1953 at Westminster Abbey.

Charles, Prince of Wales and heir to the throne of Queen Elizabeth II, speaks to guests during a reception to celebrate the 21st anniversary of Duchy originals products at Clarence House on September 11, 2013, in London.

Catherine, Duchess of Cambridge, Prince William, Duke of Cambridge, Princess Charlotte of Cambridge and Prince George of Cambridge arrive at the Church of St Mary Magdalene on the Sandringham Estate for the Christening of Princess Charlotte of Cambridge in King's Lynn, England, on July 4.

Brunei's Sultan Hassanal Bolkiah, one of the world's longest-reigning monarchs, salutes during a ceremonial guard of honor to mark his 68th birthday celebrations in Bandar Seri Begawan on August 14, 2014.

Brunei's Crown Prince Al-Muhtadee Billah Bolkiah and his new bride, Sarah, pray during their wedding banquet at the Nurul Iman Palace in Bandar Seri Begawan in September 2004.

Prince Frederik, seen here with his wife, Princess Mary, is the heir to the throne of Denmark.

Prince Naruhito, seen here with his wife, Princess Masako, is heir to the imperial throne of Japan.

Jordan's King Abdullah and Queen Rania on March 12, 2013, in Amman.

Sheikh Sabah Al-Ahmad Al-Jaber Al-Sabah has been Emir of Kuwait since 2006.

King Letsie III, seen here with Queen Masenate Mohato Seeiso, has twice become king of Lesotho -- first in 1990, when his father fled the country for five years, and again in 1996 after his father's death. Then-South African President Nelson Mandela spoke at the king's 1997 coronation.

Prince Hans-Adam II of Liechtenstein and Princess Marie-Aglae married in 1967. The prince assumed the regency of the small European nation in 1989.

Grand Duke Henri and Grand Duchess Maria Teresa of Luxembourg married in 1981. The grand duke has reigned since 2000.

Monaco's newborn royals, Princess Gabriella and Crown Prince Jacques Honore Rainier, posed for their first official photos with their parents, Prince Albert and Princess Charlene. The twins were born on December 10, 2014.

Princess Charlene of Monaco and Prince Albert II of Monaco welcomed twins Crown Prince Jacques Honore Rainier and Princess Gabriella Therese Marie in December 2014. The couple married in 2011. Prince Albert, Monaco's reigning monarch, is the son of Prince Rainier and Princess Grace, who was formerly known as Hollywood actress Grace Kelly.

King Mohammed VI of Morocco sits with his wife, Princess Lalla Salma, at the royal palace on July 13, 2002, in Rabat, Morocco.

King Willem-Alexander, seen here with his wife, Queen Maxima, succeeded his mother on the throne of the Netherlands. His mother, Princess Beatrix, was Queen of the Netherlands from 1980 to 2013, when she abdicated.

King Harald V of Norway and his wife, Queen Sonja, married in 1968.

Crown Prince Haakon of Norway is shown here with his wife, Princess Mette-Marit; son, Prince Sverre Magnus; daughter, Princess Ingrid Alexandra; and stepson, Marius Borg Hoiby.

Sultan Qaboos bin Said of Oman salutes during a military parade in 2013.

Qatar's Emir Sheikh Tamim bin Hamad Al-Thani took over leadership of the Persian Gulf nation in 2013 after the abdication of his father.

King Salman bin Abdulaziz al Saud succeeded one of his brothers, King Abdullah, on the throne of Saudi Arabia in January 2015.

King Felipe VI and Queen Letizia of Spain

Mswati III was crowned Ngwenyama, or King, of Swaziland in 1986, when he was 18. He is shown here at the White House in 2014 with Inkhosikati, or Queen, LaMbikiza, one of his 15 wives.

King of Sweden Carl XVI Gustaf and Queen Silvia attend the 2014 Nobel prize award ceremony at the Stockholm Concert Hall on December 10, 2014.

Crown Princess Victoria of Sweden attends the Nobel Prize Awards Ceremony at Concert Hall on December 10, 2014, in Stockholm. Princess Victoria is heir to the throne held by her father.

Newly crowned King Tupou VI and his wife, Queen Nanasipau'u, of Tonga proceed through the streets to the Royal Palace as part of the official coronation ceremony on July 4.