Wednesday is a big day for the British Monarchy, as this is when Queen Elizabeth II becomes the longest-reigning monarch in British history, having acceded to the throne on February 6 1952, aged 25.
Around 5.30 p.m. London time (12.30 p.m. ET), the Queen will surpass the previous record held by Queen Victoria (1837 to 1901), who ruled for 63 years and 216 days.
CNBC takes a look at some of history's other longest reigns.
– By CNBC's Alexandra Gibbs.