This equation was first developed by the Reverend Thomas Bayes in the 1700s. It calculates the probability that one event (A) is true, given that another event (B) is also true.
It is used for many purposes, including detecting faults, surveillance, military defence, search-and-rescue operations, medical screening and even spam filters.
"Its beauty arises because it underlies rational thinking and decision-making, rather than because of any intrinsic aesthetic appeal," says David Percy of the University of Salford in the UK – who could not choose between Bayes' theorem and Euler's identity.
Read more: What is the most beautiful equation?