Given the extensive research on diglossia and the many recent attempts to both refine and extend it, a review of some of these studies, especially those pertaining to the socio-economic conditions in which diglossic languages are usually embedded, seems to be warranted. It should be noted, however, that diglossia is a gradient, variable phenomenon, which cannot easily be boxed into an either-or, binary system of categorization. And as Ferguson himself recently pointed out (Ferguson 1991, in Hudson 1991a), his original formulation of diglossia was not meant to encompass all instances of multilingualism or functional differentiation of languages. Thus the many attempts to `refine' or `extend' diglossia, or to discern whether such and such is or is not a case of diglossia, may be barking up the wrong tree.
Fishman (1967) introduced the notion that diglossia could be extended to situations found in many societies where forms of two genetically unrelated (or at least historically distant ) languages occupy the H and L niches, such that one of the languages (e.g. Latin in medieval Europe), is used for religious, educational, literacy and other such prestigious domains, while another language (in the case of medieval Europe, the vernacular languages of that era) is rarely used for such purposes, being only employed for more informal, primarily spoken domains.