King of Gwynedd; eponymous hero of the Mabinogi Fourth Branch (Math fab Mathonwy); magician-king of the Welsh medieval tradition. Subject to the curious geis (taboo) that he could not live except by holding his feet in the lap of a virgin maiden — except in time of war. With his nephew Gwydion fab Dôn (one of the Children of Dôn), conducted the foundational magical-conjuration acts of the Fourth Branch: the conjuration of Blodeuwedd from the flowers of oak, broom, and meadowsweet for Lleu Llaw Gyffes's wife (after Arianrhod's third-tynged); the magical-wand virginity-test of Arianrhod that produced the Lleu-and-Dylan twin-births; the punitive deer-pig-wolf transformations of his nephews Gwydion and Gilfaethwy after their rape of Goewin (the foot-holder maiden). The Math-magician-king figure preserves a foundational Welsh medieval magic-and-kingship complex distinct from the broader Insular Celtic druid-magician tradition. The patronymic fab Mathonwy preserves an earlier-generation name (Mathonwy) attested only through this filiation; without further developmental material on Mathonwy the registry leaves Math's parentIds empty.