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 then runs westwards to the River Teme, and follows the river southeastwards through Knighton before turning south towards the River Lugg at Presteigne, which is within Wales. The boundary continues southwards across hills to the River Wye, and follows the river upstream for a short distance to Hay-on-Wye, on the Welsh side of the border. It continues southwards and rises through and across the Black Mountains, following the Hatterall Ridge past Llanthony on the Welsh side and Longtown on the English side, to reach the River Monnow near Pandy. It then generally follows the river, past Pontrilas (in England) and