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In the Vatican, some Croatian un-crowned rulers and kings, such as Duke Trpimir I and King Stjepan Držislav, were referred to as "Dux Croatorum" or "Dux Chroatorum". Historians usually translate the title Duke as "Knez", but it's still a subject of discussion since "Knez" is commonly used to designate a nobleman, but not a monarch i.e. a king.
Today the term knez is still used as the most common translation of "prince" in Bosnian, Croatian and Serbian literature. Another translation is kraljević (Slovene: kraljevič) meaning "little king" or "kingly", such as Kraljević Marko, though this term is used to refer to a prince or princess of royal birth, son or daughter of a king. Knez is nowadays a very common surname in some ex-Yugoslavian countries, mostly in Croatia, Montenegro and Serbia, but also among immigrants in Slovenia and Bosnia
The female form transliterated from Bulgarian and Russian is knyaginya (княгиня), kniahynia (княгиня) in Ukrainian, kneginja in Slovene, Croatian and Serbian (Serbian Cyrillic: кнегиња). In Russian, the daughter of a knyaz is knyazhna (княжна), in Ukrainian is kniazivna (князівна). In Russian, the son of a knyaz is knyazhich (княжич) (old form).
The title is pronounced and written similarly in different European languages. In Croatian, Bosnian and West Slavic languages, such as Polish, and Serbian, 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).
The etymology is ultimately a cognate of the English king, the German König, and the Swedish konung. The proto-Slavic form was kǔningǔ, kъnędzь, Bulgarian knyaz, East-Slavic knyaz, Polish książe, Serbian "кнез", Croatian and Slovene knez, Bosnian knjez, Czech kníže etc., as it could be a very early borrowing from the already extinct Proto-Germanic Kuningaz, a form also borrowed by Finnish and Estonian (Kuningas).
In Finnish knyaz is translated more like the word ruhtinas ("Sovereign Prince"). Ruhtinas comes from Proto-Germanic word druhtinaz, what can be translated as "warlord". In Finland tsar was officially called as suuriruhtinas after the Grand Principality of Finland. Suuriruhtinas in English means "grand prince", in Russian velikiy knyaz.
Middle Ages 
The meaning of the term changed over the course of history. Initially the term was used to denote the chieftain of a tribe. Later, with the development of feudal statehood, it became the title of a ruler of a state, and among East Slavs (Russian: княжество (kniazhestvo), Ukrainian: князівство (kniazivstvo) traditionally translated as duchy or principality), for example, of Kievan Rus'. In medieval Latin sources the title was rendered as either rex or dux.
In Bulgaria, Simeon took the title of tsar in 913. In Kievan Rus', 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 Kiev, Chernihiv, Novgorod, Pereiaslav, Vladimir-Suzdal', Muscovy, Tver, Halych-Volynia, and in the Grand Duchy of Lithuania.
As Tsardom of Russia gained dominion over much of former Kievan Rus', Velikii Kniaz Ivan IV of Russia in 1547 was crowned as Tsar. From the mid-18th century onwards, the title Velikii Kniaz was revived to refer to (male-line) sons and grandsons of Russian Emperors. See titles for Tsar's family for details.
Kniaz (Russian: Кня́зь, IPA: [ˈknʲæsʲ]) continued as a hereditary title of Russian nobility patrilineally descended from Rurik (e.g., Belozersky, Belosselsky-Belozersky, 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.
From the 18th century onwards, 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.
Finally, within the Russian Empire of 1809-1917, Finland was officially called Grand Principality of Finland (fi Suomen suuriruhtinaskunta, sv Storfurstendömet Finland, ru Velikoye Knyazhestvo Finlyandskoye).
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, the title knyaz was born by 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 градоначалник (gradonachalnik) or кмет (kmet) (Bulgaria).
See also 
|Wikimedia Commons has media related to: Knyaz|
- "князь". "Vasmer's Etymological Dictionary" online
- "knez". Oxford English Dictionary, 1989, online  (subscription required)