Diglossia

2000, Joshua A. Fishman, Chapter 3: Bilingualism with and without diglossia; diglossia with and without bilingualism, Li Wei (editor), The Bilingualism Reader, Taylor & Francis (Routledge), page 81, It is the purpose of this chapter to relate these two research traditions to each other by tracing the interaction between their two major constructs: bilingualism (on the part of psychologists) and diglossia (on the part of sociologists).

To begin with, of the two varieties involved in diglossia, the one serving (H)igh societal functions, unlike that reserved for (L)ow ones, is nobody's mother tongue: it is learned in later life largely by formal education, and is not used for ordinary conversation. Secondly, the diglossic contrast concerns widely divergent varieties, as opposed to stylistic contrasts which tend to be small-scale. Moreover, diglossia occurs within a single language, while bilingualism or multilingualism, involve far more divergent linguistic systems.

Only very small and isolated communities display neither diglossia nor bilingualism.