Seissyllwch is the name for seven cantrefs that Pryderi added to his realm, extending his rule beyond the seven cantrefs of Dyved. Its brief appearance makes the place-name a marker of territorial expansion and of Pryderi’s standing as a ruler.

Definition and Extent

The account identifies Seissyllwch with a specific group of seven cantrefs: three from Ystrad Tywi and four from the Cardigan Region. These were added to the seven cantrefs of Dyved that Pryderi already ruled, so the name distinguishes the new territory from his original domain rather than replacing it.

Pryderi’s Expansion

Pryderi’s acquisition of the cantrefs follows his succession to his father, Pwyll. He rules Dyved prosperously and is beloved by his people and those around him before adding the territories of Ystrad Tywi and the Cardigan Region. The narrative presents the expansion as an increase to an established kingship, not as a conquest described through battle or dispute.

Naming and Narrative Significance

The text says that the three cantrefs of Ystrad Tywi and four of the Cardigan Region were called the Seven Cantrevs of Seissyllwch. It does not explain the name’s linguistic origin or give further boundaries. Instead, it preserves the name through a concise account of how the seven cantrefs came together under Pryderi’s rule.