onder de rivieren

Etymology
lit. 'below the rivers', that is the Rhine and Meuse and their distributaries, which as part of the form a natural border between two larger religiocultural regions of the Low Countries (with the  forming a transitional region).

Prepositional phrase

 * 1)  In the southern Netherlands or southern Low Countries in general; to the south of the Rhine and Meuse and their distributaries, which separate the Catholic southern provinces from the northern, largely Protestant parts of the Low Countries.