The emergence of a language that can be described as specifically Cornish, rather than a dialect of late Common Brittonic, has not been conclusively dated and may have been a process lasting several hundred years.
According to Kenneth Jackson, the Common Brittonic period ended around 600 AD due to the loss of direct land communications between western and southwestern Britain following the Anglo-Saxon incursions.
[3][4] Kim McCone, however, assumes a later date, around the turn of the first millennium, citing continuing maritime connections and the various shared phonological developments during this period, such as the accent shift and internal i-affection.
[7] This task is hampered by a relative paucity of surviving texts,[8] but the existence of a number of documents written in rhyme, as well as the work of Edward Lhuyd, who visited Cornwall for three months in the early 1700s and recorded what he heard in an approximately phonetic orthography, have allowed linguists to reconstruct various stages of the phonology of the Cornish language.
Nicholas Williams dates it to before the earliest Middle Cornish texts,[18] whereas Ken George states that this change did not occur until 1600.
According to Williams, the continued writing of ⟨y⟩ and ⟨i⟩ in monosyllables is an archaism and a reflection of orthographic conservatism which does not represent the contemporary pronunciation of the scribes.
[28] Both of these interpretations are questioned by Bock and Bruch, who argue that the use of ⟨y⟩ and ⟨e⟩ in the texts reflects the phonetic reality of the language at around the time the manuscripts were written.
Jenner's system is largely based on the phonology of late Cornish, and therefore is characterised by pre-occlusion, the loss of the rounded front vowels, and the raising of /a/ to [æ].
Also Kernewek Kemmyn is characterised by phonemic consonant length, half-long vowels in stressed penultima of polysyllables where appropriate, and a number of diphthongs which are not used in other systems.
The Standard Written Form, agreed in May 2008, was developed with the intention of allowing all users of previous systems to write as they pronounce the language.
Kernowek Standard is an orthography and recommended pronunciation developed mainly by Nicholas Williams and Michael Everson in response to perceived problems with the SWF.