The article is devoted to one of aspects of changing of the political map of Baltic region. The main attention is paid to the formation of the Duchy of Courland and Semigallia. It was a result of the secularization of the possessions of the Livonian branch of Teutonic order. The formation of the duchy was a result of the outbreak of the Livonian War. The Union of Wilno was concluded on November 28, 1561. It consisted of two documents that decided the fate of the Livonian Confederation and laid the foundation of the Duchy of Courland. One of them was the so-called Pacta Subiectionis. But in this article a focus is shifted to the question of the privileges of Sigismund August (Privilegium Sigismundi Augusti). It was an important source, which became the legal basis of the Duchy of Courland and Semigallia. This document, along with the Pacta Subjectionis, in terms of the foreign policy complications caused by the Livonian war, described the relationship between the new formed political body and the Polish Crown. Duchy of Courland and Semigallia became its vassal. The document designed the system of vassal - fief relations. Privileges included the requirements to chivalry, and in the first to fourth articles concerning everything. It was about the guaranties of the Evangelical faith in accordance with the Lutheran religion, Protestant churches, orderly justice, etc. Privilege is largely confirmed the former rights of the former vassals of the Teutonic order. In addition, it established the basic principles of the transference of the land holdings. The chivalry had quite extensive rights to their fiefs. For example it concerned rights to inheritance for a family's branch-species. One of the most important points was the regulation of the procedure of recovery of charters. It confirmed and sometimes expanded the chivalric jurisdiction over the peasantry. No less important is the question of offices in the province, which could take the local knighthood. This document confirmed the right of the representatives of the chivalry, who became vassals of the Duke of Courland and Semigallia or directly of the polish king. Among them there were not only the holders of fiefs, but also the former brothers of the German order. Privilege of Sigismund Augustus formed the legal basis for the chivalry of Curland.