This thesis explores a set of legal manuscripts from medieval Wales, known as the Iorwerth recension. These manuscripts are placed in their historical and material settings, and a view of the development of the text is given. From an original thirteenth-century core, a story is told of the recension’s development across a range of further rearrangements and uses across the Middle Ages. The use of the Iorwerth text is analysed for what it might reveal of the intentions and mindset of the copyist. In the second half of the thesis, a more traditional textual account is given of the Iorwerth recension. Volume II contains tables and editions to accompany the arguments in the first volume.