The title of Grand Duke (Latin, Magnus Dux; German, Großherzog, Russian, Великий князь) used in Slavic, Baltic, and Germanic countries, is ranked in honour below King but higher than a sovereign Duke (Herzog) or Prince (Fürst). The feminine form is Grand Duchess. A Grand Duke's territory is called a Grand Duchy.
The term probably originated in Germany, where it was used to refer to the rulers of medieval Russian states. The title Magnus Dux or Grand Duke (Didysis kunigaikštis in Lithuanian) was used by the rulers of Lithuania, and after Jagiello also became kings of Poland and was later found among the titles used by kings of the Polish-Lithuanian Commonwealth. In 1582 king John III of Sweden added Grand Duke of Finland to the subsidiary titles of the Swedish kings, however without any factual consequences, Finland already being a part of the Swedish realm. The Polish kings of the Swedish Vasa dynasty also used the grand ducal title for their non-Polish territory. After the Russian conquests, it continued to be used by the Russian Emperor in his role as ruler of Lithuania (1793-1918) and of autonomous Finland (1809-1917) as well. The Holy Roman Empire ruling house of Habsburg instituted a similar Grand Duchy in Transylvania in 1765.
Further, Grand Duke is the translated form of the title Megas Doux, used in the Byzantine Empire during the Palaeologian dynasty (1259-1453).
Byzantine Grand Dukes
The Latin title dux was rendered δουξ in Greek, and was a common title for imperial generals in the Late Roman Empire. In the Eastern Empire, a dux ranked just below a strategos. Under the Byzantine theme system, the commander of the theme was styled a dux. The title Megas Dux (Grk. Μέγας Δουξ) first appears in the Comnenian period, and was conferred upon the admiral of the Byzantine navy. Among the recipients of this honor was Roger de Flor of the Catalan Company, who was given the title for his services against the Turks during the reign of Andronicus II
Russian Grand Dukes
"Grand Duke" is the traditional translation of the title Velikiy Kniaz, which from the 11th century was at first the title of the leading Prince of Kievan Rus', then of several princes of the Rus'. From 1328 the Velikii Kniaz of Muscovy appeared as the Grand Duke for "all of Russia" until Ivan IV of Russia in 1547 was crowned as Tsar. Thereafter the title was given to sons and grandsons (through male lines) of the Tsars and Emperors of Russia. The daughters and paternal granddaughters of Russian Emperors, as well as the consorts of Russian Grand Dukes, were generally called "Grand Duchesses" in English.
A more accurate translation of the Russian title would be Great Prince—especially in the pre-Petrine era—but the term is neither standard nor widely used in English. In German, however, a Russian Grand Duke was known as a Großfürst, and in Latin as Magnus Princeps.
Most often, a reigning Grand Duke or Duchess was styled Royal Highness. Other members of the families differed in style. Junior members of the Grand Ducal Family of Luxembourg are also Royal Highnesses; however, this derives from their status as cadet members of the dethroned royal house of Bourbon-Parma and not from the Grand Ducal title.
In Hesse-Darmstadt and Baden, however, junior members of the dynasty bore the style of Grand Ducal Highness (Großherzogliche Hoheit). For instance, prior to her marriage, Empress Alexandra of Russia was known as "Her Grand Ducal Highness Princess Alix of Hesse and by Rhine" (Ihre Großherzogliche Hoheit Alix Prinzessin von Hessen bei Rhein).
A Russian Grand Duke or Grand Duchess was an Imperial Highness.