In church law, the union of churches (unio ecclesiarum) concerned the merger of two and more dioceses under the same bishop. In the Middle Ages, canonists were already pointing to three types of union: 1) aeque principalis; 2) unio per subiectionem, when one of the churches was subject to the other and thus the episcopal dignity remained only in that one, and finally, the third kind, called 3) unio per extinctionem, when two particular churches, usually dioceses, were merged into a single new one. The canonical achievements in the field of union of churches and benefices were collected and summarized, among others, in the treatise De unionibus ecclesiarum atque beneficiorum by Nicolaus Thilen, and in the work of Anaclet Reiffenstuel entitled Ius canonicum universum. The three types of union of churches and benefices presented above, distinguished by their mergers, were adopted into the Code of Canon Law of 1917 (canons 1419 and 1420). The 450th anniversary of the union concluded on July 1, 1569 in Lublin was celebrated in 2019. As a result of this union the Kingdom of Poland, called the Crown, merged with the Grand Duchy of Lithuania. The canonical models of the union of churches and benefices, developed in medieval canon law, are important for a closer description of the essence of this relationship, starting with the first of them, i.e. the union concluded in 1385 in Krevo. The political relationships established between the Kingdom of Poland and the Grand Duchy of Lithuania largely corresponded to the three canonical models of the church union indicated above, i.e. unio aeque principalis (1385), unio per subiectionem (1413) and unio per extinctionem seu translationem (1569).