Seisyllwg () was a

petty kingdom A petty kingdom is a kingdom described as minor or "petty" (from the French 'petit' meaning small) by contrast to an empire or unified kingdom that either preceded or succeeded it (e.g. the numerous kingdoms of Anglo-Saxon England unified into t ...

of medieval

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

.Davies, p. 85 It is unclear when it emerged as a distinct unit, but according to later sources it consisted of the former Kingdom of Ceredigion plus the region known as Ystrad Tywi. Thus it covered the modern county of

Ceredigion Ceredigion (), historically Cardiganshire (, ), is a Principal areas of Wales, county in the West Wales, west of Wales. It borders Gwynedd across the River Dyfi, Dyfi estuary to the north, Powys to the east, Carmarthenshire and Pembrokeshire t ...

, part of

Carmarthenshire Carmarthenshire (; or informally ') is a Principal areas of Wales, county in the South West Wales, south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. ...

, and the

Gower Peninsula The Gower Peninsula (), or simply Gower (), is a peninsula in the South West Wales, south-west of Wales. It is the most westerly part of the historic county of Glamorgan, and is now within the City and County of Swansea. It projects towards th ...

. It is evidently named after Seisyll, king of Ceredigion 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 centre of power for

Hywel Dda Hywel ap Cadell, commonly known as Hywel Dda, which translates to Howel the Good in English, was a Welsh king who ruled the southern Welsh kingdom of Deheubarth and eventually came to rule most of Wales. He became the sole king of Seisyllw ...

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

Kingdom of Dyfed The Kingdom of Dyfed (), one of several Welsh petty kingdoms that emerged in 5th-century sub-Roman Britain in southwest Wales, was based on the former territory of the Demetae (modern Welsh ''Dyfed''). The royal line was founded by Irish ...

to form the new kingdom of

Deheubarth Deheubarth (; , thus 'the South') was a regional name for the Welsh kingdoms, realms of south Wales, particularly as opposed to kingdom of Gwynedd, Gwynedd (Latin: ''Venedotia''). It is now used as a shorthand for the various realms united under ...

Origins

It is unclear when Seisyllwg emerged as a distinct unit. It is assumed to have been named for Seisyll ap Clydog, King of Ceredigion in the 7th or early 8th century, and as such he is traditionally regarded as its founder.Lloyd, p. 257 and note. Seisyll appears in the

Harleian genealogies __NOTOC__ The Harleian genealogies are a collection of Old Welsh genealogies preserved in British Library, Harley MS 3859. Part of the Harleian Library, the manuscript, which also contains the '' Annales Cambriae'' (Recension A) and a version of ...

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.Koch, p. 1602. The name Seisyllwg appears in some later sources, such as the Book of Llandaff, the Welsh Triads, and the Welsh laws, the latter of which describes it as one of the three principal subdivisions of South Wales, along with Morgannwg and ''Reinwg'' (probably

Dyfed Dyfed () is a preserved county in southwestern Wales, covering the modern counties Carmarthenshire, Ceredigion and Pembrokeshire. It is mostly rural area with a coastline on the Irish Sea and the Bristol Channel. Between 1974 and 1996, Dyfed w ...

). However, the first clear description of the territory is in the First Branch of the Mabinogi, where Seisyllwg is said to include the four

cantref A cantref ( ; ; plural cantrefi or cantrefs; also rendered as ''cantred'') was a Wales in the Early Middle Ages, medieval Welsh land division, particularly important in the administration of Welsh law. Description Land in medieval Wales was divid ...

s of Ceredigion plus the three of Ystrad Tywi, 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 Mawr (Rhodri the Great) of

Gwynedd Gwynedd () is a county in the north-west of Wales. It borders Anglesey across the Menai Strait to the north, Conwy, Denbighshire, and Powys to the east, Ceredigion over the Dyfi estuary to the south, and the Irish Sea to the west. The ci ...

, who became steward over Gwgon's realm. While this gave Rhodri no standing to claim kingship himself, he was able to install his and Angharad's younger son, Cadell, as the new King of Seisyllwg. Cadell ruled as a vassal to his father, and later, to his elder brother Anarawd, who established the House of Dinefwr. After Cadell's death in 911, Seisyllwg was divided among his two sons,

Hywel Hywel (), sometimes anglicised as Howel or Howell, is a Welsh masculine given name. It may refer to: Monarchs * Hywel the Great, late fifth- and early sixth-century noble in Brittany, saint and king of Brittany in the Arthurian legend. *Hywel ap R ...

(later known as Hywel Dda, or Howel the Good), and Clydog.Lloyd, p. 333.Koch, p. 945. Hywel probably already had control over the neighbouring

kingdom of Dyfed The Kingdom of Dyfed (), one of several Welsh petty kingdoms that emerged in 5th-century sub-Roman Britain in southwest Wales, was based on the former territory of the Demetae (modern Welsh ''Dyfed''). The royal line was founded by Irish ...

by that time; there are no known kings of Dyfed following the death of Llywarch ap Hyfaidd 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 Seisyllwg to Hywel. Hywel quickly merged Seisyllwg and Dyfed into the new kingdom of

, which covered most of southwest Wales. From this power base, he later went on to unite almost all of Wales.

Notes

References

*Davies, John (2007). ''A History of Wales (Hanes Cymru)''. Penguin Books. . *Jones, Basil (1851)
"Vestiges of the Gael in Gwynedd"
''Archaeologia Cambrensis'', pp. 1–86. W. Pickering. * * {{Rulers of medieval Wales 920 disestablishments Kingdoms of Wales 680 establishments