Being a collection of various medieval manuscripts setting out the descent of sundry kings of Wales.
When for example, Julius Caesar assumed power in Rome he did so equipped with a suitable genealogy, showing his descent from the goddess Venus to demonstrate his suitability to rule. The Welsh kings naturally enough, followed this Roman practice. (As did the Anglo-Saxon kings, for whom almost every genealogy features the god Woden somewhere along the line.)
It also helps knowing that in Welsh medieval society status or braint, depended almost entirely on parentage and the ability to recite one's authentic Welsh ancestors back to the nth degree. Wales was therefore a nation of genealogists in many ways.
These are basically the historical sources from which much of the information about the kings or tywysogion that ruled the kingdoms of medieval Wales, although in some cases the attribution of specific lineages to specific kingdoms is a matter of tradition and guesswork.
And one does have to bear in mind that firstly, the manuscripts contain genealogical information that do not necessarily tally with one another, because, secondly people were not averse to a exercising a certain amount of license and invention.
The genealogies themselves are contained in a number of medieval manuscripts, the most important of which are detailed below.
A: The Harleian genealogies
Which are the earliest surviving document preserved in a single manuscript Harleian MS 3859, bound together with what is known as the 'A' version of the Annales Cambriae. The manuscript itself dates from around 1100, but as the genealogies begin with Owain ap Hywel Dda it is generally presumed that they were originally compiled during his reign (that is between 950 and 987) but are obviously based on oral recollections of a much older date.
B: The Jesus College genealogies
These are contained in the Jesus College MS 20, folios 33r to 41r and draws on the same body of material, but also contains some variations and additions which suggests that it was not directly derived from the Harleian MS. However, the Jesus College manuscript is dated to the late fourteenth century but was probably compiled in the thirteenth century. It is therefore of a much later date than the Harleian genealogies and is therefore considered to be less reliable.
You can incidentally view an image of the actual Jesus College MS 20 at http://image.ox.ac.uk/show?collection=jesus&manuscript=ms20
C: Bonedd Gwyr y Gogledd
The Bonedd Gwyr y Gogledd, that is the 'Pedigrees of the Men of the North' is a collection found in a number of manuscripts, the earliest being Peniarth MS.45 dated to the second half of the thirteenth century. It overlaps with both the previously mentioned Harleian and Jesus College genealogies, but often with discrepancies.
D: Bonedd y Sant
Or the 'Pedigrees of the Saints' since most of the early Christian Welsh saints were allegedly of royal descent. There are a total of two dozen different manuscripts ranging in date from the early thirteenth century to the late seventeenth century, although the material is much older in origin and was probably compiled at St Davids or Mynyw.
The Bonedd y Saint should not be confused with the late medieval Achau'r Saint , which is a simply a corrupt copy of the former.
E: The Mostyn genealogy
Or the Mostyn MS. 117 which dates from the late thirteenth century which records the genealogy of Llywelyn ap Gryffudd and that of a certain Arthur
F: The Brychan documents
Contained in the MS British Library Cotton Domitian and also known as the Cognacio Brychan, it is an account of Brychan, ruler of Brycheiniog in south Wales, and his progeny; copied by Sir John Prise in the sixteenth century from a now-lost manuscript that had perhaps been written in the thirteenth century.
An example drawn from the Harleian MS for the genealogy for Owain ap Hywel Dda a tenth century king of Deheubarth,
Ouen map Higuel map Catell map Rotri map Mermin map Etthil merch Cinnan map Rotri map Iutguaul map Catgualart map Catgollaun map Catman map Iacob map Beli map Run map Mailcun map Catgolaun Iauhir map Eniaun girt map Cuneda map Aetern map Patern pesrut map Tacit map Cein map Guorcein map Doli map Guordoli map dumn map Gurdumn map Amguoloyt map Anguerit map Oumun map dubun map Brithguein map Eugein map Aballac map Amalach, qui fuit Beli magni filius et Anna mater eius quam dicunt esse consobrina mariae uirginis matris d’ni n’ri ih’u xp’i.
'map' being the Old Welsh for 'son of' (Modern Welsh 'ap') and cognate
with the Gaelic
Which tells us that
Owain is the son of Hywel son of Cadell, son of Rhodri, son of Merfyn son of Eithyl, daughter of Cynan. son of Rhodri, son of Idwal, son of Cadwalladr son of Cadwallon son of Cadfan, son of Iago, son of Beli, son of Rhun, son of Maelgwyn, son of Cadwallon Lawhir, son of Einion, son of Cunedda, etc all the way back to 'Beli magni' (Beli mawr or the great Celtic god Beli) and Anna, who they say was the cousin of the Virgin Mary mother of our Lord Jesus Christ
Which, one would think, was a reasonable enough pedigree for any Welsh
king; demonstrating descent from the family of Jesus of Nazareth
The website http://www.kmatthews.org.uk/history/texts.html has a comprehensive online database of many of these Welsh geneologies.