What have been the two most common names of Popes?

Upon election, a new pontiff chooses a regnal name, often to honor a predecessor or a significant saint. The name John has been selected more than any other, used by 23 popes throughout history. This count includes a fascinating historical quirk: there was never a Pope John XX. An 11th-century numbering error involving an antipope led Pope John XXI to skip the number, believing he was correcting the official sequence. The most recent was the beloved Pope John XXIII, who convened the Second Vatican Council.

The second most frequent choice is Gregory, with 16 popes bearing the name. Derived from the Greek for "watchful," it is a fitting title for a spiritual leader. Many popes named Gregory left a major mark on the world. Gregory I, known as Gregory the Great (Review), was a pivotal figure in the early Middle Ages and is the namesake of the Gregorian chant. Centuries later, Pope Gregory XIII was responsible for the 1582 reform that gave the world the Gregorian calendar, the system still in use today.