The province stretched along the Soča/Isonzo River, from its source at Mt. Jalovec in the Julian Alps down to the Gulf of Trieste near Monfalcone. In the northwest, the Predil Pass led to the Duchy of Carinthia, in the northeast Mts. Mangart, Razor and Triglav marked the border with the Duchy of Carniola (Upper Carniola).