John XXI seems a bit out of order; did we miss John XX and John XIX? As usual this kind of mixup is due to some confusion with antipopes; when you strike an antipope from the list of valid popes, it's difficult to keep everybody's numbers in order. John XXI yielded to the advice of contemporary historians and just chose the next undisputed number.

His reign was a short one, also passed in Viterbo. He was elected on September 13, 1276, and almost immediately started building a new wing of the papal palace. He was too excited to see it finished, however, and moved in before the building was sound. Part of it fell in on top of him while he was asleep and gave him a mortal wound to the head; he died on May 20, 1277.