Which living monarch has been on the throne the longest?

Explanation

Bhumibol Adulyadej, also known as Rama IX (and within the Thai royal family and to close associates simply as Lek) has reigned since 9 June 1946, and as such, he is the world's longest-serving current head of state and the longest-reigning monarch in Thai history. The second-longest is Queen Elizabeth II, who has been on the throne since 6 February 1952.