The megas doux was one of the highest positions in the hierarchy of the later Byzantine Empire, denoting the commander-in-chief of the Byzantine navy. It is sometimes also given in English by the half-Latinizations megaduke or megadux. The Greek word δούξ is the Hellenized form of the Latin term dux, meaning leader or commander.
The megas doux Alexios Apokaukos in the garb of his office
Grand duke is a European hereditary title, used either by certain monarchs or by members of certain monarchs' families. In status, a grand duke traditionally ranks in order of precedence below an emperor, King, grand prince, archduke, or prince-archbishop, and above a sovereign prince or sovereign duke. The title is used in some current and former independent monarchies in Europe, particularly:in the present-day Grand Duchy of Luxembourg
historically by the sovereigns of former independent countries, such as Tuscany
in Baden, Hesse, Oldenburg, Mecklenburg-Schwerin, Mecklenburg-Strelitz and Saxe-Weimar – grand duchies from 1815 to 1918, and all now part of present-day Germany
formerly also in some countries in Northern Europe, such as the Grand Duchy of Finland or the Grand Duchy of Lithuania
1860 postage stamp of the Grand Duchy of Finland
Portrait of Grand Duchess Maria Fiodorovna by Heinrich von Angeli (1874) Saint Petersburg, Hermitage Museum