Sociolinguistics
As an aspect of study of the relationships between codes and social structure, diglossia is an important concept in the field of sociolinguistics. At the social level, each of the two dialects has certain spheres of social interaction assigned to it and in the assigned spheres it is the only socially acceptable dialect (with minor exceptions). At the grammatical level, differences may involve pronunciation, inflection, and/or syntax (sentence structure). Differences can range from minor (although conspicuous) to extreme. In many cases of diglossia, the two dialects are so divergent that they are distinct languages as defined by linguists: they are not mutually intelligible.
The dialect which is the original mother tongue is almost always held in low esteem; it is of low prestige. Its spheres of use involve informal, interpersonal communication: conversation in the home, among friends, in marketplaces. In some diglossias, this vernacular dialect is virtually unwritten. Those who try to give it a literature may be severely criticized or even persecuted. The other dialect is held in high esteem and is devoted to written communication and formal spoken communication, such as university instruction, primary education, sermons, and speeches by government officials. It is usually not possible to acquire proficiency in the formal, "high" dialect without formal study of it. Thus in those diglossic societies which are also characterized by extreme inequality of social classes, most people are not proficient in speaking the high dialect, and if the high dialect is grammatically different enough, as in the case of Arabic diglossia, then these uneducated classes cannot understand most of the public speeches they might hear on television and radio. The high prestige dialect (or language) tends to be the more formalised, and its forms and vocabulary often 'filter down' into the vernacular, though often in a changed form.
In many diglossic areas there is controversy and polarization of opinions of native speakers regarding the relationship between the two dialects and their respective statuses. In cases where the "high" dialect is objectively not intelligible to those exposed only to the vernacular, some people insist that the two dialects are nevertheless a common language. The pioneering scholar of diglossia, Charles A. Ferguson, observed that native speakers proficient in the high prestige dialect will commonly try to avoid using the vernacular dialect with foreigners and may even deny its existence, even though the vernacular is the only socially appropriate one for them themselves to use when speaking to their relatives and friends. Yet another common attitude is that the low dialect—which is everyone's native language—ought to be abandoned in favor of the high dialect, which presently is nobody's native language.
Read more about this topic: Diglossia