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Written in the mid 14th century, around 1350, the book is the earliest Welsh prose anthology, though it also contains examples of early Welsh poetry.

Although it was originally a single volume, the collection is now bound as two separate volumes, known as Peniarth MS 4 and Peniarth MS 5. It was presented to the National Library of Wales as one of its founding collections, after being purchased by Sir John Williams in 1904.

Peniarth MS 4 contains the earliest copies of the Welsh tales collectively known as the Mabinogion: the Four Branches of the Mabinogi; Culhwch and Olwen; The Dream of Macsen Wledig; Lludd and Llefelys; Peredur; Owain (also known as The Lady of the Fountain); and Geraint and Enid.

MS 5, originally the first part of the full manuscript, is made up of religious texts, translated into Welsh from the original Latin.

The script takes its name from Rhydderch ab Ieuan Llwyd (c. 1325-1400), who was from Parcrhydderch in the Ceredigion parish of Llangeitho. It is believed that the book was copied for him in the mid-14th century.

Rhydderch was part of a dynasty with a lengthy tradition of literary patronage, and is believed to have been an authority on Welsh law. Five scribes are thought to have worked on the manuscripts at the Strata Florida Abbey, a former Cistercian abbey situated just outside Pontrhydfendigaid, near Tregaron.

Much like the Red Book of Hergest, the manuscript is thought to take its name from its original owner and the colour of its first binding.