Often mistakenly equated with the Rheingau, the Gau was based around the former Roman administrative district of Civitas Mattiacorum.
The Gau probably had already been created by the beginning of the reign of Charlemagne (after 771) as the private property of the Frankish king and his heirs.
The administration of the Königssondergau lay in the hands of the gau counts, whose royal court (curtis) stood near today's Hessian Parliament in Wiesbaden.
Over time parts were also given to the Archdiocese of Mainz (e.g. Oestrich, Geisenheim, Rüdesheim am Rhein and Lorch in the Veronese donation of 983 by Emperor Otto II) or sold to other owners.
Also, the Lords of Eppstein penetrated by exploiting bailiwick rights, purchase and inheritance in the Königssondergau, where they became opponents of the Counts of Nassau.