The map of English habitation names in the 14th century provides a clue about the expansion of English-speaking. It seems that English spread along the southern flank of Bodmin Moor into the countryside around Bodmin, which, because of its status, may have seen a relatively early growth of the English language. Other evidence for the linguistic state of Cornwall from the Normans’ arrival to the early 1300s is sketchier. There are no documentary statements from this period on the condition of the language or where it was spoken. Our only evidence lies in placenames, Fortunately, they provide some useful clues, as these were the centuries during which certain sound changes occurred in the Cornish language. With some delay, these would show up in the spelling of placenames. The spelling changes can therefore be used as a proxy for Cornish-speaking communities.
The most relevant change was that known as assibilation, when -t or -d at the end of elements became -s. This is supposed to have occurred first in words ending in -nt, so that nant (valley) became nans, pont (bridge) became pons etc. Changes in words such as bod (cottage) or coyt (wood) came a little later. The timing of these change is critical but unfortunately uncertain. The Old Cornish Vocabulary, a Cornish-Latin word-list of 960 words, is the first extended example of written Cornish and is usually dated to around 1100, although it may be somewhat later. In it, the Cornish for valley is spelt nans, bridge is spelt pons and wind is guins. However, words that later changed to -s, like cuit (wood), luit (grey) or davat (sheep) retained the -t. This bears out the conclusion that the change from -nt to -ns took place earlier than -t to -s where the -t immediately followed a vowel. So, allowing for some time lapse, a map of the 14th century examples of nant/nans and gwint/gwins placenames ought to tell us where Cornish was still spoken in the mid to late 1100s.
This map suggests that even this early, a large area of east Cornwall did not experience this sound change. There were some isolated instances of the change near Liskeard and St Teath, but the map suggests that Cornish speaking communities were rare east of the Bodmin district perhaps as early as the mid-12th century.
A similar map for the element for wood (cuit/coyt/coys) supports this conclusion. The division between the -t and -s endings is slightly further west than for nant/gwint. But when had the sound change occurred? In the 11 examples of places with this element and with spelling examples old enough to show the change from coyt to coys, the median year when -s was first recorded was 1284. This might suggest that the change to -s in this word occurred in the early to mid-1200s. If this is so the placename evidence implies the area where Cornish was spoken had shrunk further since the mid-1100s. Indeed, this may even suggest that Cornish had been pushed back west of the Camel-Fowey line at this stage. It’s likely that socio-economic change – the period from the 11th century to the early 14th saw population growth, the emergence of market towns and the colonisation of the uplands by farmers – lay behind the linguistic transformation of east Cornwall.