... The royal name thus introduced became popular among the crown vassals, and Isabelle of Angouleme, betrothed to Hugues de Lusignan, but married to King John, brought Isabel to England, whence her daughter, the wife of Friedrich II., conveyed Isabella to Germany and Sicily. (History of Christian Names, Yonge, 1884)

    Meliadus was king of Leonois, or Lionesse, a country famous in the annals of romance, which adjoined the kingdom of Cornwall, but has now disappeared from the map, having been, it is said, overwhelmed by the ocean.  Meliadus was married to Isabella, sister of Mark, king of Cornwall... (Age of Chivalry, Bulfinch, 1884)

ISABEL, ISABELLA.  According to some this name is the same as Jezebel, which Tregelles thinks may mean "without cohabitation," i.e. αλοχος, Plat. p. 249 B, chaste, modest; and he compares it with Agnes.  Bailey says "Isabella, q.d. Eliza Bella, handsome Elizabether;" but Isabella is rather from Isabél, the Spanish rendering of Elizabeth.  A correspondent of N. Q. 4th S. iii. 516, says he has written proofs that one and the same person, the celebrated Isabella Clara Eugenia, daughter of Philip II., whilst she signed "Isabel, Assabel," had coins and medals struck with "Albert and Elisabet," and diplomatic seals with "Isabella, D. G. HISPAN. INFANS."  The nicknames are Nib and Nibbie." (Prænomina, Charnock, 1882)