Prince William looks on as Prince George greets the queen in July 2015 at Sandringham in England. Image Source: Getty / Chris Jackson [1]

In honor of Queen Elizabeth of Britain's 90th birthday [2] in April, her family agreed to do a rare round of interviews about her life and legacy for a BBC film called Our Queen at 90. The documentary features personal footage of the family, as well as candid conversations with Prince William; his wife, Kate; Prince Charles; and many more of their relatives.

Among the many sweet things revealed about the queen was something in relation to her great-grandson Prince George, William and Kate's young son. According to Kate, Prince George calls her majesty by the nickname "Gan-Gan." [3] Kate's admission sent the internet into a tizzy, as royal fans around the world reacted to the sweet — and surprisingly normal — level of informality between the royal relations.

It turns out, though, that there is a deeper, more meaningful reason behind George's nickname for the queen.

"Gan-Gan" is a designation that generations of royals have used to describe their great-grandmothers. According to royal biographer Kitty Kelley, Prince Charles used the term to refer to his great-grandmother Queen Mary, the mother of Queen Elizabeth's father King George VI. Princes William and Harry actually called the queen's mother, Queen Elizabeth the Queen Mother, "Gan-Gan" until her death in 2002. Harry even let his nickname for his great-grandmother slip during his own cameo in Our Queen at 90 [4].

If there's one thing we know, it's that the royals love tradition [5]. And George's sweet nickname for the queen is the latest in a long line of royal customs.

Princes William and Harry with their mother, Princess Diana, and great-grandmother Queen Elizabeth the Queen Mother in June 1998 at London's Buckingham Palace. Image Source: Getty / Anwar Hussein [6]

Related:
• How Will and Kate Are Keeping Royal Parenting Traditions Alive [7]
• Everything You Want to Know About Prince George's Posh Preschool [8]
• Why Queen Elizabeth II Once Cried Over a Ship [9]