Along with Welsh and Breton, Cornish is descended directly from the Common Brittonic language spoken throughout much of Britain before the English language came to dominate. It was the main language of Cornwall for centuries until it was pushed westwards by English, maintaining close links with its sister language Breton with which it was mutually intelligible until well into the Middle Ages. Cornish continued to function as a common community language in parts of Cornwall until the late 18th century, and continued to be spoken in the home by some families into the 19th and possibly 20th centuries, overlapping the beginning of revival efforts. A process to revive the language was begun in the early 20th century, with a number of orthographical systems still in use, although an attempt was made to impose a Standard Written Form in 2008. In 2010 UNESCO announced that its former classification of the language as "extinct" was "no longer accurate".
Since the revival of the language, some Cornish textbooks and works of literature have been published and an increasing number of people are studying the language. Recent developments include Cornish music, independent films, and children's books. A small number of people in Cornwall have been brought up to be bilingual native speakers, and the language is taught in many schools. The first Cornish language crèche opened in 2010.
Cornish is one of the Brittonic languages, which constitute a branch of the Insular Celtic section of the Celtic language family. Brittonic also includes Welsh, Breton and the Cumbric language; the last is extinct. Scottish Gaelic, Irish and Manx are part of the separate Goidelic branch of Insular Celtic.
Joseph Loth viewed Cornish and Breton as being two dialects of the same language, claiming that "Middle Cornish is without doubt closer to Breton as a whole than the modern Breton dialect of Quiberon is to that of Saint-Pol-de-Léon ."
Cornish evolved from the Common Brittonic spoken throughout Britain south of the Firth of Forth during the British Iron Age and Roman period. As a result of westward Anglo-Saxon expansion, the Britons of the southwest were separated from those in modern-day Wales and Cumbria. Some scholars have proposed that this split took place after the Battle of Deorham in about 577. The western dialects eventually evolved into modern Welsh and the now extinct Cumbric, while Southwestern Brittonic developed into Cornish and Breton, the latter as a result of emigration to parts of the continent, known as Brittany over the following centuries.