It is bounded on the north-west by the province of Cantabria, north-east by the province of Biscay, south by La Bureba, south-east by Ebro, south-west by Páramos, and on the east by the province of Álava.
The geographical boundaries of this region are defined by its adjacency with the provinces of Cantabria, Biscay, La Bureba, Ebro, Páramos, and Álava.