The Königssondergau, first mentioned in a document in 819, referred to an administrative unit of Frankish royal possessions in today's Rhine-Main region. The northern border of the "Gau" ("pagus Kuningessundere") was roughly formed by the Limes, while the southern border was formed by the Rhine and Main rivers. In the west, the Gaugrenze ran along the Hohe Straße and the Wallufbach, in the east roughly along the Dais and the Kriftel (Schwarzbach).

Mechtildshausen was the district's place of jurisdiction and Wiesbaden the administrative seat, where the Gaugraf resided. Through fiefs, donations and sales, parts of the originally closed imperial estate complex passed to other owners, including the Counts of Nassau, the Lords of Eppstein and various monasteries. Adjacent districts were Nidda- and Maingau to the east, Oberrheingau to the south of the Main and (Unter-)Rheingau to the west.