Illustrious Highness
In modern times Serene Highness ("Altesse Sérénissime") is used as the equivalent of the German "Durchlaucht". In the 17th century it became the general style borne by the heads of the reigning princely states of the Holy Roman Empire ("reichsständische Fürsten"), as "Illustrious Highness ("Erlaucht") became customary for those of the comital houses ("reichsständische Grafen", i.e. Counts of the Empire). In 1825 the Imperial German Diet agreed to grant the style "Durchlaucht" to the heads of all mediatized princely houses domiciled in Germany elevated to the rank of "Fürst" are also styled Durchlaucht. In 1829 the style of Erlaucht, which had formerly been borne by the reigning Counts of the empire, was similarly granted to the mediatized countly families (Almanach de Gotha, 1909, 107).