The number XX was skipped due to confusion over John XIV (983–984), which resulted from an error in the textual transmission of his entry in the Liber Pontificalis.
However, "Popes John XV" through "XIX" are dual-numbered "XVI" through "XX" because some sources had already used the altered numbering sequence.
The Pisan antipope Alexander V was succeeded by John XXIII, who abdicated in 1415 as part of a deal to end the schism.
The last three popes of the schism were listed as Gregory XII (1406–1409), Alexander V (1409–1410), and John XXIII (1410–1415).
"[8] This is reflected in modern editions of the Annuario Pontificio, which extend Gregory XII's reign to 1415.