CHICAGO — Queen Elizabeth II passed away Thursday at the age of 96, after being Britain’s longest-serving monarch and the only sovereign most Britons have ever known.

The Queen made a visit to Chicago as the only stop in the U.S. on her Great Lakes tour in 1959.

The British monarchy’s rules state that “a new sovereign succeeds to the throne as soon as his or her predecessor dies.” That means Queen Elizabeth II’s eldest son, Prince Charles, became king immediately upon her death.