Linguist Kenneth H. Jackson has suggested that the evolution in syllabic structure and sound pattern was complete by around AD 550, and labelled the period between then and about AD 800 "Primitive Welsh".Koch, p. 1757. This Primitive Welsh may have been spoken in both Wales and the ('Old North') – the Brittonic-speaking areas of what are now northern England and southern Scotland – and therefore may have been the ancestor of Cumbric as well as Welsh. Jackson, however, believed that the two varieties were already distinct by that time. The earliest Welsh poetry – that attributed to the or "Early Poets" – is generally considered to date to the Primitive Welsh period. However, much of this poetry was supposedly composed in the , raising further questions about the dating of the material and language in which it was originally composed. This discretion stems from the fact that Cumbric was widely believed to have been the language used in Hen Ogledd. An 8th-century inscription in Tywyn shows the language already dropping inflections in the declension of nouns.
