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Deheubarth (‘the south part’), one of Wales's larger medieval kingdoms. Formed during the reign of Hywel Dda (died 949/50) by combining, through marriage, Seisyllwg and Dyfed, it covered the west and south-west of Wales and sometimes extended into Brycheiniog. Hywel's line ended with Rhys ap Tewdwr's death (1093) in the early years of Norman advances. The Welsh victory near Cardigan (1136) enabled his son Gruffydd to recover part of the kingdom; but the invaders were never expelled completely from the south (Dyfed), even by Rhys ap Gruffydd, the Lord Rhys (d. 1197), under whom Deheubarth was one of the three great kingdoms of 12th-cent. Wales. Its capital, Dinefwr, acquired legendary status, and Rhys's achievements enhanced Deheubarth's traditional significance. It disintegrated after his death, and the princes of Gwynedd extended their influence over northern Deheubarth until it was replaced (1277–87) by Edward I's new counties of Cardigan and Carmarthen; the marcher lordships of Pembrokeshire and south Carmarthenshire continued to occupy the southern portion. The descendants of Deheubarth's rulers declined into obscurity. The name and concept of Deheubarth has not been revived, unlike Dyfed and Ceredigion.
Ralph Alan Griffiths