The expression 'south Wales' is not officially defined, and its meaning has changed over time.
Between the Statute of Rhuddlan of 1284 and the Laws in Wales Act 1535, crown land in Wales formed the Principality of Wales. This was divided into a Principality of South Wales and a Principality of North Wales. The southern principality was made up of the counties of Ceredigion and Carmarthenshire, areas that had previously been part of the Welsh kingdom of Deheubarth ('the southern land'). The legal responsibility for this area lay in the hands of the Justiciar of South Wales based at Carmarthen. Other parts of southern Wales were in the hands of various Marcher Lords.
The Laws in Wales Acts 1542 created the Court of Great Sessions in Wales based on four legal circuits. The Brecon circuit served the counties of Brecknockshire, Radnorshire and Glamorgan while the Carmarthen circuit served Cardiganshire, Carmarthenshire and Pembrokeshire. Monmouthshire was attached to the Oxford circuit for judicial purposes. These seven southern counties were thus differentiated from the six counties of north Wales.
The Court of the Great Sessions came to an end in 1830, but the counties survived until the Local Government Act 1972 which came into operation in 1974. The creation of the county of Powys merged one northern county (Montgomeryshire) with two southern ones (Breconshire and Radnorshire).
There are thus different concepts of south Wales. Glamorgan and Monmouthshire are generally accepted by all as being in south Wales. But the status of Breconshire or Carmarthenshire, for instance, is more debatable. In the western extent, from Swansea westwards, local people might feel that they live in both south Wales and west Wales. Areas to the north of the Brecon Beacons and Black Mountains are generally considered to be in Mid Wales.
A further point of uncertainty is whether the first element of the name should be capitalized: 'south Wales' or 'South Wales'. As the name is a geographical expression rather than a specific area with well-defined borders, style guides such as those of the BBC and The Guardian use the form 'south Wales'.