Hay-on-Wye the north-easternmost tip of the Brecon Beacons National Park, just north of the Black Mountains. The town is just on the Welsh side of the border with Herefordshire, England, here defined by the Dulas Brook. Where the brook joins the River Wye just north of the town, the border continues northwards along the river. The Wye was the boundary between the former counties and districts of Radnorshire and Brecknockshire. The adjacent village of Cusop lies on the English side of the Dulas Brook. The nearest city is Hereford, county town of Herefordshire, some to the east. The Royal Mail depot
England–Wales border Shrewsbury, and Hugh d'Avranches at Chester, creating a new expansionist earldom in each case. In the Domesday Book of 1086, Norman lands are recorded west of the Wye at Chepstow and Caldicot in the Gwent Levels (); over the whole of north east Wales as far west as the River Clwyd, an area known to the Welsh as the "Perfeddwlad"; and west of Offa's Dyke, especially in Powys where a new castle was named, after its lord, Montgomery. Domesday Book no doubt records the extent of English penetration into Wales. This suggests that Offa's Dyke still approximately represented the boundary