It is Peniarth 29 of the National Library of Wales, and deals with legal and historical matters.
The Black Book of Chirk was one of the collection of manuscripts amassed at the mansion of Hengwrt, near Dolgellau, Gwynedd, by Welsh antiquary Robert Vaughan (c. 1592 – 1667); the collection later passed to the newly established National Library of Wales as the Peniarth or Hengwrt-Peniarth Manuscripts.
John Gwenogvryn Evans claimed it was the oldest surviving manuscript of Welsh law, dating it to 1220, and he published a facsimile in 1909.
It is now regarded as a derivative work, a partial copy of the Iorwerth Redaction in one manuscript line, and somewhat later in the century.
[1] Some of the historical content, such as the details of tradition relating to the sixth-century dynast Mordaf Hael, is not now regarded as significant factually.