It is a commonplace of modern Linguistics that language boundaries are sharpest and older forms most likely to survive in areas which, for one reason or another, have been communicatively isolated and where populations have remained stable over time. When barriers to communication break down, it is said, rapid language change takes place and dialect boundaries become muted. Development of transportation routes, large scale population movements, increasing social mobility, centralization of education and government facilities, universal exposure to the language of mass media and the need to master expository styles of science and bureaucracy, along with many other factors characteristic of ongoing urbanization, all generate powerful pressures for linguistic uniformity. Yet, while it is true that indices of regional and social speech diversity have undergone far-reaching change, especially in the metropolitan centers of modern industrial states most directly exposed to these pressures, many important dialect differences remain and show no signs of disappearing.

Frequency or intensity of communication is perhaps a necessary precondition for the disappearance of dialect boundaries, but it is by no means sufficient. We know of many areas in Europe where people in adjoining villages speak mutually intelligible dialects which are nonetheless set off by clear speech distinctions. In these localities members of one community regularly communicate with members of the other, but speakers use their own locally specific forms. To adopt the other's way of speaking would count as discourteous and constitute a breach of local etiquette. Similar language usage conventions were also observed in a long term ethnographic study of dialect distribution, social organization and interaction patterns in a North Indian village where many different caste groups have lived side by side for several centuries.