Welsh law, the native law of Wales. Although increasingly superseded by English law after the 13th century, Welsh law has been preserved in lawbooks that represent important documents of medieval Welsh prose.

The traditional name given to Welsh law is Cyfraith Hywel, or Law of Howel. Howel Dda (910–950), the Welsh king after whom the law is named, was likely responsible for some consolidation of the law about the middle of the 10th century, though no extant manuscript dates from his reign. The oldest extant Welsh lawbook is a manuscript in Latin dating from about 1200, and about a dozen manuscripts in Welsh date from the 13th or early 14th century.

The Welsh lawbooks were compilations made by practicing lawyers, though they were also used for instruction. A few seem to be casual collections of miscellaneous material, but most purport to give a complete statement of the law. These “complete” manuscripts fall into three groups, generally called the Book of Iorwerth, the Book of Blegywryd, and the Book of Cyfnerth. The oldest manuscripts are those of the Book of Iorwerth, though the Book of Cyfnerth—which is attributed to Morgenau and his son Cyfnerth, members of the most famous family of lawyers in Gwynedd—reflects the earliest stage of development. The Book of Blegywryd resembles that of Cyfnerth but shows strong ecclesiastical influence, and it has now been shown to be a translation from a Latin compilation that can be compared with the so-called Leges Henrici Primi (Law of Henry I), which was written in England at the beginning of the 12th century.

Medieval Welsh lawbooks contain several strata: some provisions already obsolete when they were written, other traditional material that was still living law, and other more or less recent innovations. Thus, in the Book of Iorwerth most of the opening section on the court—which gives more prominence to the officers of the chase who were so significant in the heroic age than to the administrative officers who in fact guarded royal interests—was obsolete in the 13th century. In the land law, however, a detailed account of the procedure for claiming land shows that what had been a nonjudicial mode of taking possession of land had been transformed into a possessory action comparable to the assize of novel disseisin in England. In the last sections of the book, there is a very practical statement of the rules for compensation for cattle trespass and for the contract of joint plowing, whose importance increased greatly in the 13th century.