ARCHDUKE, a title peculiar in modern times to the Austrian royal family (Lat. archidux, Ger. Erzherzog). The designation was exceedingly rare during the Middle Ages. The title of "arch duke palatine" (Pfalz-Erzherzog) was assumed first by Duke Rudolph IV. (died 1365). Rudolph IV. used the title on his seals and charters till he was compelled to desist by the Emperor Charles IV. The title did not legally belong to the house of Habsburg until 1453, when the Emperor Frederick III. conferred the title of archduke of Austria on his son Maximilian and his heirs.