[1] 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.

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.

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.

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.

Hywel quickly merged Seisyllwg and Dyfed into the new kingdom of Deheubarth, which covered most of southwest Wales.