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Coordinates: Ystrad Tywi (English: Vale of [the river] Tywi) is an area of south-west Wales situated on the banks of the Tywi river as it approaches the sea to join the Bristol Channel at Carmarthen. Although Ystrad Tywi was never a kingdom itself, it was a valuable territory and was fought over by the various kings of Dyfed, Deheubarth, Seisyllwg, the Kingdom of Gwynedd, the Kingdom of Morgannwg and the Normans. The area now makes up most of the modern county of Carmarthenshire.
At the start of the 8th century Ystrad Tywi was part of the kingdom of Dyfed. Around the year 730 Seisyll ap Clydog, king of Ceredigion, captured Ystrad Tywi from Rhain ap Cadwgan, king of Dyfed, and annexed it to his own kingdom. The name Seisyllwg was given to the new enlarged kingdom.
In 894 Ystrad Tywi and Ceredigion were laid waste by King Anarawd of Gwynedd together with an English force from his ally King Alfred, in an attempt to regain the lands previously held by his father Rhodri Mawr.
In 920 Hywel Dda united Seisyllwg and Dyfed to create the kingdom of the Deheubarth.
The last of these cantrefi was lost to the Normans in the 11th century, and for the rest of the Middle Ages only the other two cantrefi formed Ystrad Tywi.
Ystrad Tywi was transformed into the county of Carmarthen when Edward I enacted the Statute of Rhuddlan in 1284 following the success of his war against Llywelyn ap Gruffudd, Prince of Wales. The commotes of Kidwelly, Iscennen and Carnwillion and part of Dyfed were added later to form the modern county.
In the first branch of the Four Branches of the Mabinogi, the tale Pwyll Pendefig Dyfed is told of the son of Pwyll, Pryderi, following his father on the throne of Dyfed and adding the three cantrefi of Ystrad Tywi and the four cantrefi of Ceredigion to his kingdom.