The numbering of popes named John does not occur in strict numerical order. Although there have been twenty-one legitimate popes named John, the numbering has reached XXIII because of two clerical errors that were introduced in the Middle Ages: first, antipope John XVI was kept in the numbering sequence instead of being removed; then, the number XX was skipped because Pope John XXI counted John XIV twice.
The numbering of popes named John does not occur in strict numerical order. Although there have been twenty-one legitimate popes named John, the numbering has reached XXIII because of two clerical errors that were introduced in the Middle Ages: first, antipope John XVI was kept in the numbering sequence instead of being removed; then, the number XX was skipped because Pope John XXI counted John XIV twice.
Source: Wikipedia "Pope John numbering" · CC BY-SA 4.0
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