Ferguson's (1959) Concept of Diglossia
Modelled on the French diglossie, the original concept of diglossia goes back to Charles Ferguson (1959). According to Ferguson (1959), the H-variety and the L-variety have to be two divergent forms of the same language which are above the level of a standard-with-dialects distinction, but which stay below the level of two separate (related or unrelated) languages.
Characteristically, the H-variety is never used for everyday conversation and in this respect a diglossic situation differs from a standard-with-dialects situation in which the standard may also be used for everyday conversation.
The original description of diglossia according to Ferguson (1959) is:
"DIGLOSSIA is a relatively stable language situation in which, in addition to the primary dialects of the language (which may include a standard or regional standards), there is a very divergent, highly codified (often grammatically more complex) superposed variety, the vehicle of a large and respected body of written literature, either of an earlier period or in another speech community, which is learned largely by formal education and is used for most written and formal spoken purposes but is not used by any sector of the community for ordinary conversation."
(In Wei 2000: 75)