This article views the act of Union issued at Mielnik in 1501 in the longue durée of Lithuano-Polish relations from 1385/86. It stresses the need to treat the dynastic aims of Jogaila and his son Casimir seriously, regarding the thrones of Lithuania and Poland within the context of developments in family law and inheritance in the Grand Duchy and the Kingdom in the fifteenth century. Textual echoes are noted between a ‘union’ agreement outlined ca. 1453 and the Mielnik document.