The Union of Lublin (Lithuanian - Liublino unija, Polish - Unia lubelska), signed July 1, 1569, in Lublin, Poland, marked the creation of a single State Polish-Lithuanian, a Commonwealth ruled by a single elected monarch carrying the duties of Polish King and Grand Duke of Lithuania with a common Senate and parliament (the Sejm). The Union was a step forward to protect the kingdoms from potential wars with Russia. The Union of Lublin replaced a prior personal union of the Kingdom of Poland and the Grand Duchy of Lithuania in an elective monarchy, since Sigismund II Augustus, the last of the Jagiellons, remained heirless after three marriages. Moreover, the autonomy of Royal Prussia was largely abandoned. While the Union of Lublin has been viewed by Polish historians as a voluntary creation of states union, Lithuanian historians have pointed out the Union as a pacific domination by Poland.

Map Source: University of Texas at Austin, Perry-Castaneda Library, Map Collection
http://www.lib.utexas.edu/maps/
From "The Cambridge Modern History Atlas" edited by Sir Adolphus William Ward, G.W. Prothero, Sir Stanley Mordaunt Leathes, and E.A. Benians. Cambridge University Press; London. 1912