Royal fans are puzzled over the unusual nickname Prince William is said to use for his younger brother, Prince Harry.

Born on 15 September 1984, Harry was officially named Prince Henry Charles Albert David Wales by his parents, King Charles III, then the Prince of Wales, and Diana, Princess of Wales. From the beginning, however, Diana made it clear her son would be known as Harry. Over the years, the name stuck so firmly that many forget it is not his given name.

But Harry himself has claimed that William does not call him Henry or Harry — he calls him Harold. The question of why has sparked endless speculation.

The mystery was raised on Reddit when one user asked: “I’m listening to Spare, and [William] is always calling him Harold. His given name is Henry. Why?”