A466 road about long and runs through the counties of Herefordshire and Gloucestershire in England, and Monmouthshire in Wales. It runs south from Kingsthorne, on the A49 south east of Hereford to Monmouth, crossing the A40. After crossing the River Wye at the Wye Bridge, Monmouth and Bigsweir Bridge near Llandogo, it follows a picturesque route south down the Wye valley through Tintern and Chepstow to the M48 motorway at junction 2. The road crosses the border between England and Wales at three places along its length: at Buckholt, Redbrook, and Bigsweir. It runs through an Area of Outstanding Natural Beauty, and
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