The Union of Grodno established a real union

Real union

Real union is a union of two or more states, which share some state institutions as in contrast to personal unions; however they are not as unified as states in a political union...

 between the Grand Duchy of Lithuania

Grand Duchy of Lithuania

The Grand Duchy of Lithuania was a European state from the 12th /13th century until 1569 and then as a constituent part of Polish-Lithuanian Commonwealth until 1791 when Constitution of May 3, 1791 abolished it in favor of unitary state. It was founded by the Lithuanians, one of the polytheistic...

 and the Duchy of Livonia

Duchy of Livonia

The Duchy of Livonia was a territory of the Grand Duchy of Lithuania — and later the Polish–Lithuanian Commonwealth — that existed from 1561 to 1621...

 on 25 December 1566, during the Livonian War

Livonian War

The Livonian War was fought for control of Old Livonia in the territory of present-day Estonia and Latvia when the Tsardom of Russia faced a varying coalition of Denmark–Norway, the Kingdom of Sweden, the Union of the Grand Duchy of Lithuania and the Kingdom of Poland.During the period 1558–1578,...

. Livonia had submitted itself to Sigismund II Augustus

Sigismund II Augustus

Sigismund II Augustus I was King of Poland and Grand Duke of Lithuania, the only son of Sigismund I the Old, whom Sigismund II succeeded in 1548...

 by the Treaty of Vilnius (1561)

Treaty of Vilnius (1561)

The Treaty of Vilnius or Vilna was concluded on 28 November 1561, during the Livonian War, between the Livonian Confederation and the Polish-Lithuanian Commonwealth at Vilnius...

. Livonia's administrative division was re-organized with its castellans becoming members of the Lithuanian senate. The union did not impact Livonian jurisdiction, which was to be carried out according to its traditional customs.