Rhwng Gwy a Hafren

A region of medieval Wales, located in the Welsh Marches between Powys to the north and Brycheiniog to the south, and bounded by the rivers Gwy and Hafren.
The Rhwng Gwy a Hafren included the regions Buellt, Gwyrtheyrnion, Maelienydd, Arwystli, Elenydd, Cedewain, Elfael, and the northern reaches of Brycheiniog. 
Also: "Between Wye and Severn"