Rhwng Gwy a Hafren

Rhwng Gwy a Hafren (English: Between Wye and Severn) was a region of medieval Wales, located in the Welsh Marches between Powys to the north and Brycheiniog to the south. It was bounded by the rivers Wye (Welsh: Gwy) and Severn (Welsh: Hafren). It covered about the same territory as Radnorshire, now part of the county of Powys. The region first came into its own in the 9th or 10th centuries, when it was ruled by leaders who operated independently of the surrounding kingdoms. After the Norman invasion, it comprised the central part of the Welsh Marches and was the site of frequent struggles between Welsh and Norman forces.

The name Rhwng Gwy a Hafren appears in various medieval lists of cantrefs and commotes, and is rendered in Latin in the works of Gerald of Wales. The name means "between the Wye and the Severn", and those two rivers were its most important boundaries. However, the territories associated with it are not always consistent: generally, the lists include the cantrefs of Elfael and Maelienydd and the commote of Gwrtheyrnion. The cantref of Buellt is also often associated with the region, despite being located across the Wye, and there is some conjecture that may tie in Arwystli as well. The small commotes of Ceri in Maelienydd and Cwmwd Deuddwr north of Buellt (and also across the Wye) were located in this area and appear in some sources, but are not included in the lists of divisions.

...
Wikipedia