Llawysgrif Pomffred presents for the first time an edition of an overlooked Welsh law manuscript, Peniarth 259B. This is an important and groundbreaking edition which will contribute to our understanding of the relationship and development of the Welsh law texts. The manuscript contains a law text of the Cyfnerth redaction, seen to be the earliest of the Welsh law redactions, and it also has a lengthy tail of additional material which is largely practical in nature, and seems to reflect the legal situation in the March of Wales, with English and Welsh legal customs being mixed. The manuscript may have been given to a certain Einion ab Adda whilst he was in prison in Pontefract.