Middle Welsh in the context of "Gildas"

Proto-Celtic, or Common Celtic, is the reconstructed ancestral proto-language of all known Celtic languages, and a descendant of Proto-Indo-European. It is not attested in writing but has been partly reconstructed through the comparative method. Proto-Celtic is generally thought to have been spoken between 1300 and 800 BC, after which it began to split into different languages. Proto-Celtic is often associated with the Urnfield culture and particularly with the Hallstatt culture. Celtic languages share common features with Italic languages that are not found in other branches of Indo-European, suggesting the possibility of an earlier Italo-Celtic linguistic unity.

Proto-Celtic is currently being reconstructed through the comparative method by relying on later Celtic languages. Though Continental Celtic presents much substantiation for Proto-Celtic phonology, and some for its morphology, recorded material is too scanty to allow a secure reconstruction of syntax, though some complete sentences are recorded in the Continental Gaulish and Celtiberian. So, the main sources for reconstruction come from Insular Celtic languages with the oldest literature found in Old Irish and Middle Welsh, dating back to authors flourishing in the 6th century AD.

Y Gododdin (Welsh: [əː ɡɔˈdɔðɪn]) is a medieval Welsh poem consisting of a series of elegies to the men of the Brittonic kingdom of Gododdin and its allies who, according to the conventional interpretation, died fighting the Angles of Deira and Bernicia at a place named Catraeth in about AD 600. It is traditionally ascribed to the bard Aneirin and survives only in one manuscript, the "Book of Aneirin".

The Book of Aneirin manuscript is from the later 13th century, but Y Gododdin has been dated to between the 7th and the early 11th centuries. The text is partly written in Middle Welsh orthography and partly in Old Welsh. The early date would place its oral composition soon after the battle, presumably in the Hen Ogledd ("Old North"); as such it would have originated in the Cumbric dialect of Common Brittonic. Others consider it the work of a poet from Wales in the 9th, 10th, or 11th century. Even a 9th-century date would make it one of the oldest surviving Welsh works of poetry.

Gofannon (Welsh pronunciation: [ɡɔˈvanɔn]) is a figure in Middle Welsh literature as a great metal worker and as the son of Dôn. His name can be compared with the Old Irish gobae (gen. gobann) ‘smith’, Middle Welsh / Cornish / Breton gof (pl. gofein) ‘smith’, Gaulish gobedbi ‘with the smiths’, all of which are cognate with Lithuanian gabija ‘sacred home fire’, gabus ‘gifted, clever’. His apparent counterpart in Irish mythology, Goibniu, in addition to his duties as a smith, also takes on the role of a divine hero who brewed an ale of immortality, in addition to being an architect and builder.

In Welsh mythology, Gofannon killed his nephew, Dylan Ail Don, not knowing who he was. One of the tasks given to Culhwch if he were to win the hand of Olwen was to get Gofannon to sharpen his brother Amaethon's plough.

The Mabinogion (Welsh pronunciation: [mabɪˈnɔɡjɔn] ) is a collection of the earliest Welsh prose stories, compiled in Middle Welsh in the 12th–13th centuries from earlier oral traditions. There are two main source manuscripts, created c. 1350–1410, as well as a few earlier fragments. Often included in the broader mythologies described as the Matter of Britain, the Mabinogion consists of eleven stories of widely different types, offering drama, philosophy, romance, tragedy, fantasy and humour. Strictly speaking, the Four Branches of the Mabinogi are the main sequence of related tales, but seven others include a classic hero quest, "Culhwch and Olwen"; a historic legend, complete with glimpses of a far off age, in "Lludd and Llefelys"; and other tales portraying a very different King Arthur from the later popular versions.

The stories were created and amended by various narrators over a very long period of time, and scholars beginning from the 18th century predominantly viewed the tales as fragmentary pre-Christian Celtic mythology, or folklore. Since the 1970s, an investigation of the common plot structures, characterisation, and language styles, especially in the Four Branches of the Mabinogi, has led to an understanding of the integrity of the tales, and they are now seen as a sophisticated narrative tradition, both oral and written, with ancestral construction from oral storytelling, and overlay from Anglo-French influences.

Old Welsh (Welsh: Hen Gymraeg) is the stage of the Welsh language from about 800 AD until the early 12th century when it developed into Middle Welsh. The preceding period, from the time Welsh became distinct from Common Brittonic around 550, has been called "Primitive" or "Archaic Welsh".

The Book of Taliesin (Welsh: Llyfr Taliesin) is one of the most famous of Middle Welsh manuscripts, dating from the first half of the 14th century though many of the fifty-six poems it preserves are taken to originate in the 10th century or before.

The volume contains some of the oldest poems in Welsh, possibly but not certainly dating back to the sixth century and to a real poet called Taliesin.

In Arthurian legend, Kay /ˈkeɪ/ (Welsh: Cai, Middle Welsh Kei or Cei; Latin: Caius; French: Keu; Old French: Kès or Kex) is King Arthur's foster brother and later seneschal, as well as one of the first Knights of the Round Table. In later literature he is known for his acid tongue and bullying, boorish behaviour, but in earlier accounts he was one of Arthur's premier warriors. Along with Bedivere, with whom he is frequently associated, Kay is one of the earliest characters associated with Arthur. Kay's father is called Ector in later literature, but the Welsh accounts name him as Cynyr Ceinfarfog.