Seisyllwg was a petty kingdom

Petty kingdom

A petty kingdom is one of a number of small kingdoms, described as minor or "petty" by contrast to an empire or unified kingdom that either preceded or succeeded it...

 of medieval Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. It is unclear when it emerged as a distinct unit, but according to later sources it consisted of the former Kingdom of Ceredigion

Kingdom of Ceredigion

The Kingdom of Ceredigion was one of several Welsh kingdoms that emerged in 5th-century post-Roman Britain. Its area corresponded roughly to that of the modern county of Ceredigion. The kingdom's hilly geography made it difficult for foreign invaders to conquer. Cardigan Bay bordered to the west...

 plus the region known as Ystrad Tywi

Ystrad Tywi

Ystrad 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...

. Thus it covered the modern county of Ceredigion

Ceredigion

Ceredigion is a county and former kingdom in mid-west Wales. As Cardiganshire , it was created in 1282, and was reconstituted as a county under that name in 1996, reverting to Ceredigion a day later...

, part of Carmarthenshire

Carmarthenshire

Carmarthenshire is a unitary authority in the south west of Wales and one of thirteen historic counties. It is the 3rd largest in Wales. Its three largest towns are Llanelli, Carmarthen and Ammanford...

, and the Gower Peninsula

Gower Peninsula

Gower or the Gower Peninsula is a peninsula in south Wales, jutting from the coast into the Bristol Channel, and administratively part of the City and County of Swansea. Locally it is known as "Gower"...

. It is evidently named after Seisyll

Seisyll

Seisyll ap Clydog was King of Ceredigion in Wales some time in the late 7th or early 8th century. He gave his name to the later kingdom of Seisyllwg, which consisted of Ceredigion plus the region known as Ystrad Tywi; as such he was possibly the king responsible for the expansion...

, king of Ceridigion in the 7th or early 8th century, but it is unknown if he was directly responsible for its establishment. In the 10th century Seisyllwg became the center of power for Hywel Dda

Hywel Dda

Hywel Dda , was the well-thought-of king of Deheubarth in south-west Wales, who eventually came to rule Wales from Prestatyn to Pembroke. As a descendant of Rhodri Mawr, through his father Cadell, Hywel was a member of the Dinefwr branch of the dynasty and is also named Hywel ap Cadell...

, who came to rule most of Wales. In 920 Hywel merged Seisyllwg with the Kingdom of Dyfed

Kingdom of Dyfed

The Kingdom of Dyfed is one of several Welsh petty kingdoms that emerged in 5th-century post-Roman Britain in south-west Wales, based on the former Irish tribal lands of the Déisi from c 350 until it was subsumed into Deheubarth in 920. In Latin, the country of the Déisi was Demetae, eventually to...

 to form the new kingdom of Deheubarth.

Origins

It is unclear when Seisyllwg emerged as a distinct unit. It is assumed to have been named for Seisyll

Seisyll

Seisyll ap Clydog was King of Ceredigion in Wales some time in the late 7th or early 8th century. He gave his name to the later kingdom of Seisyllwg, which consisted of Ceredigion plus the region known as Ystrad Tywi; as such he was possibly the king responsible for the expansion...

, King of Ceredigion in the 7th or early 8th century, and as such he is traditionally regarded as its founder. Seisyll appears in the Harleian genealogies

Harleian genealogies

The Harleian genealogies are a collection of Old Welsh genealogies preserved in British Library, Harleian MS 3859. Part of the Harleian Collection, the manuscript, which also contains the Annales Cambriae and a version of the Historia Brittonum, has been dated to c. 1100, although a date of c.1200...

 for the Kings of Ceredigion, but no early sources attribute the foundation of Seisyllwg to him, and the name Ceredigion continues to be used into the 9th century. The name Seisyllwg appears in some later sources, such as the Book of Llandaff

Book of Llandaff

The Book of Llandaff is a 12th century compilation of documents relating to the history of the diocese of Llandaff in Wales...

, the Welsh Triads

Welsh Triads

The Welsh Triads are a group of related texts in medieval manuscripts which preserve fragments of Welsh folklore, mythology and traditional history in groups of three. The triad is a rhetorical form whereby objects are grouped together in threes, with a heading indicating the point of likeness...

, and the Welsh law

Welsh law

Welsh law was the system of law practised in Wales before the 16th century. According to tradition it was first codified by Hywel Dda during the period between 942 and 950 when he was king of most of Wales; as such it is usually called Cyfraith Hywel, the Law of Hywel, in Welsh...

s, the latter of which describes it as one of the three principal subdivisions of South Wales, along with Morgannwg and Reinwg (probably Dyfed

Kingdom of Dyfed

The Kingdom of Dyfed is one of several Welsh petty kingdoms that emerged in 5th-century post-Roman Britain in south-west Wales, based on the former Irish tribal lands of the Déisi from c 350 until it was subsumed into Deheubarth in 920. In Latin, the country of the Déisi was Demetae, eventually to...

). However, the first clear description of the territory is in the First Branch of the Mabinogi

Mabinogion

The Mabinogion is the title given to a collection of eleven prose stories collated from medieval Welsh manuscripts. The tales draw on pre-Christian Celtic mythology, international folktale motifs, and early medieval historical traditions...

, where Seisyllwg is said to include the four cantrefs of Ceredigion plus the three of Ystrad Tywi

Ystrad Tywi

Ystrad 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...

, a description which accords with that in the laws.

Later history and merger with Dyfed

In 872, Gwgon, the last in the traditional line of kings of Ceredigion, drowned, leaving no heir. Gwgon's sister, Angharad, was married to Rhodri the Great

Rhodri the Great

Rhodri the Great was King of Gwynedd from 844 until his death. He was the first Welsh ruler to be called 'Great', and the first to rule most of present-day Wales...

 of Gwynedd

Kingdom of Gwynedd

Gwynedd was one petty kingdom of several Welsh successor states which emerged in 5th-century post-Roman Britain in the Early Middle Ages, and later evolved into a principality during the High Middle Ages. It was based on the former Brythonic tribal lands of the Ordovices, Gangani, and the...

, who became steward over Gwgon's realm. While this gave Rhodri no standing to press a claim to kingship himself, he was able to install his and Angharad's younger son, Cadell

Cadell ap Rhodri

Cadell ap Rhodri was the son of Rhodri Mawr . He inherited the kingdom of Seisyllwg from his mother Angharad of Seisyllwg in 878, and passed it to his son, Hywel Dda , on his death in 909. Cadell and Hywel had previously conquered Dyfed in 904/905, establishing Hywel as the king in that region...

, as the new King of Seisyllwg. Cadell ruled as a vassal to his father, and later, to his elder brother Anarawd

Anarawd ap Rhodri

Anarawd ap Rhodri was a King of Gwynedd, also referred to as "King of the Britons" by the Annales Cambriae.Anarawd's father Rhodri the Great had eventually become ruler of most of Wales, but on his death in 878 his kingdom was shared out between his sons, with Anarawd inheriting the throne of...

, who established the Dinefwr family.

After Cadell's death in 911, Seisyllwg was divided among his two sons, Howel

Hywel Dda

Hywel Dda , was the well-thought-of king of Deheubarth in south-west Wales, who eventually came to rule Wales from Prestatyn to Pembroke. As a descendant of Rhodri Mawr, through his father Cadell, Hywel was a member of the Dinefwr branch of the dynasty and is also named Hywel ap Cadell...

 (later known as Hywel Dda, or Howel the Good), and Clydog. Hywel probably already had control over the neighboring kingdom of Dyfed

Kingdom of Dyfed

The Kingdom of Dyfed is one of several Welsh petty kingdoms that emerged in 5th-century post-Roman Britain in south-west Wales, based on the former Irish tribal lands of the Déisi from c 350 until it was subsumed into Deheubarth in 920. In Latin, the country of the Déisi was Demetae, eventually to...

 by that time; there are no known kings of Dyfed following the death of Llywarch ap Hyfaidd

Llywarch ap Hyfaidd

Llywarch ap Hyfaidd was the king of Dyfed, Wales until its conquest in 904 or 905 by Cadell ap Rhodri and his son Hywel Dda, rulers of Seisyllwg. Kingship passed briefly to his brother, Rhodri, until rule was consolidated by Hywel. Hywel married Llywarch's daughter, Elen, to legitimise his claim...

 in 904, and Hywel is known to have been married to Llywarch's daughter, Elen. He certainly had control over it by the time Clydog died in 920, leaving the whole of Seissylwg to Hywel. Hwyel quickly merged Seisyllwg and Dyfed into the new kingdom of Deheubarth, which covered most of southwest Wales. From this power base, he later went on to unite almost all of Wales.