The Megas Mesazon or Lord High Treasurer is the third highest rank of the Great Officers of the Realm, in Diadochia below the Megas Quaestor and above the Megas Proedros tou Mystikoú Symvoulíou. The Megas Mesazon functions as the head of His Imperial and Royal Majesty's Treasury. Since the 17th century the office has often been held, not by a single person, but placed in commission, normally headed by the Prime Minister of Diadochia as First Lord of the Treasury.