The title is pronounced and written similarly in different Eastern European languages. In West Slavic languages, such as Croatian, Polish, and Sorbian, the word has later come to denote "lord", and in Czech, Polish and Slovak also came to mean "priest" (kněz, ksiądz, kňaz) as well as "duke" (knez, kníže, książę, knieža).
As the degree of centralization grew, the ruler acquired the title Velikii Kniaz (Великий Князь) (translated as Grand Prince or Grand duke, see Russian Grand Dukes). He ruled a Velikoe Knyazhestvo (Великое Княжество) (Grand Duchy), while a ruler of its vassal constituent (udel, udelnoe kniazhestvo or volost) was called udelny kniaz or simply kniaz.
When Kievan Rus' became fragmented in the 13th century, the title Kniaz continued to be used in East Slavic states, including Novgorod, Vladimir-Suzdal', Muscovy, Tver, Halych-Volynia, and in the Grand Duchy of Lithuania.
As Muscovy gained dominion over much of former Kievan Rus', Velikii Kniaz Ivan IV of Russia in 1547 was crowned as Tsar. Since the mid-18th century, the title Velikii Kniaz has been revived to allude to sons and grandsons (through male lines) of the Russian Emperors. See titles for Tsar's family for details.
Kniaz continued as a hereditary title of Russian nobility patrilineally descended from Rurik (e.g., Repnin, Gorchakov) or Gediminas (e.g., Galitzine, Troubetzkoy). Members of Rurikid or Gedyminid families were called princes when they ruled tiny quasi-sovereign medieval principalities. After their demesnes were absorbed by Muscovy, they settled at the Moscow court and were authorised to continue with their princely titles.
Since 18th-century, the title was occasionally granted by the Tsar, for the first time by Peter the Great to his associate Alexander Menshikov, and then by Catherine the Great to her lover Grigory Potemkin. After 1801, with the incorporation of Georgia into the Russian Empire, various titles of numerous local nobles were controversially rendered in Russian as "kniazes". Similarly, many petty Tatar nobles asserted their right to style themselves "kniazes" because they descended from Genghis Khan.
See also "Velikiy Knyaz" article for more details.
In the 19th century, the Serbian term knez (кнез) and the Bulgarian term knyaz (княз) were revived to denote semi-independent rulers of those countries, such as Alexander Karađorđević and Alexander of Battenberg. Prior to Battenberg, only two Bulgarian rulers had born the title knyaz: Boris I and his son Simeon I during the First Bulgarian Empire (9th-10th century). At the height of his power, Simeon adopted the title of tsar ("emperor"), as did the Bulgarian rulers after the country became officially independent in 1908.
As of Bulgaria's independence in 1908, Knyaz Ferdinand became Tsar Ferdinand, and the words knyaz/knyaginya began to be used instead for the tsar's children – the heir to the throne, for example, held the title Knyaz Tarnovski ("Knyaz of Tarnovo").
In parts of Serbia and western Bulgaria, knez was the informal title of the elder or mayor of a village or zadruga until around the 19th century. Those are officially called gradonačelnik (Serbia) and gradonachalnik or kmet (Bulgaria).