Until 1974, Monmouthshire, also formerly known as the County of Monmouth, was an administrative county in the south-east of Wales, on the border with England, and later classed as one of the thirteen historic counties of Wales. Its area now corresponds approximately to the present principal areas of Monmouthshire, Blaenau Gwent, Newport and Torfaen, and those parts of Caerphilly and Cardiff east of the Rhymney River.

The England–Wales border, sometimes referred to as the Wales–England border or the Anglo-Welsh border, runs for 160 miles (260 km) from the Dee estuary, in the north, to the Severn estuary in the south, separating England and Wales.