Known in the Welsh as Trioedd Ynys Prydein, the Triads of the Island of Britain, these are a grouping of material in threes recounting events from Welsh history and mythology. They served as an index or device to jog the memory adopted by the cyfarwyddion, the storytellers of medieval Wales. The tales that stood behind the Triads being preserved, at that time, only in the memory of the cyfarwyddion and the retelling of the tales themselves.

The Triads are now preserved in some of the principle early manuscripts of Welsh literature, namely,

The earliest of these documents are the Peniarth Manuscript and the Black Book of Carmarthen from the 13th century, but the earliest Triads themselves are believed to be from the 9th or 10th centuries.

However the versions of the Triads preserved tend to differ between the various manuscripts, (as naturally they would, being derived from an oral tardition) so there does not seem to a definitive list of Triads; set out below are as many as I could find.