The term diglossie (French) was first coined (as a translation of Greek diglossía 'bilingualism') by the Greek linguist and demoticist Jean (Iannis, Yannis) Psichari (Psycharis). The Arabist William Marçais used the term in 1930 to describe the linguistic situation in Arabic-speaking countries. In Charles Ferguson's article "Diglossia" in the journal 'Word' (1959) diglossia was described as a kind of bilingualism in a given society in which one of the languages is (H), i.e. has high prestige, and another of the languages is (L), i.e. has low prestige. In Ferguson's definition, (H) and (L) are always closely related. Fishman also talks about diglossia with unrelated languages: "extended diglossia" (Fishman 1967), for example Sanskrit as (H) and Kannada as (L) or Alsatian (Elsassisch) in Alsace as (L) and French as (H). In some cases, the nature of the connection between (H) and (L) is disputed, for example Jamaican Patois as (L) and Standard English as (H) in Jamaica. This case also points to another problem: while Patois does indeed have less phonemes than Standard English, it has two tonemes: it would for this and other reasons be wrong to ascertain that Patois is less complex than Standard English. The same can be said of many (L) Languages: some Swiss German dialects have /e/ /E/ and /{/ while Standard German only has /e/ and /E/. Jamaican Patois has less vowel phonemes than standard Englishes, but it has additional palatal /k_j/ and /g_j/ phonemes. (H) is usually the written language whereas (L) is the spoken language. Kloss distingusihes between exoglossia (as in Alsace) and endoglossia (as in German-speaking Switzerland). In formal situations, (H) is used, in informal situations, (L) is used.
Especially in endoglossia the (L) form may also be called "basilect", the (H) form "acrolect" and an intermediate form "mesolect". Note however that there is no "mesolect" in German-speaking Switzerland and in Luxembourg. Whether Paraguay has a form of diglossia is controversial. Guaraní and Spanish are both official languages of Paraguay. Some scholars argue that there are Paraguyans who actually don't speak Guaraní. See Guarani language
Ferguson's classic examples include Standard German/Swiss German, Standard Arabic/vernacular Arabic, Standard French/Kréyòl in Haiti, and Katharevousa/Dhimotiki in Greece. However, Kréyòl is now recognised as a standard language in Haiti. Swiss German dialects are hardly languages with low prestige in Switzerland; and colloquial Arabic has more prestige than standard Arabic nowadays (see Chambers, Sociolinguistic Theory). And after the end of the military regime, Dhimotiki was made into Greece's only standard language (1976). Nowadays, Katharevousa is no longer used. Harold Schiffman writes about Swiss German: "it seems to be the case that Swiss German was once consensually agreed to be ina diglossic hierarchy with Standard German, but that this consensus is now breaking." There is also a lot of code switching especially in the Arabic world; according to Andrew Freeman this is "different from Ferguson's description of diglossia which states that the two forms are in complementary distribution." To a certain extent, there is code switching and overlap in all diglossic societies, even German-speaking Switzerland. Furthermore, in Ferguson's definition, diglossia is not bilingualism; however this depends on the scholar's definition of language. For example, different kinds of Arabic are not mutually intelligible; even though many are, but this may also be due to exposure to different varieties rather than inherent linguistic properties.
Examples where the High/Low dichotomy is justified in terms of social prestige include Italian dialects as (L) and Standard Italian as (H) in Italy and German dialects and standard German in Germany. In Italy and Germany, those speakers who still speak dialects typically use dialect in informal situations, especially in the family. In German-speaking Switzerland, on the other hand, Swiss German dialects are to a certain extent even used in schools and to a larger extent in churches. Ramseier calls German-speaking Switzerland's diglossia a "medial diglossia", whereas Felicity Rash prefers "functional diglossia". Paradoxically, Swiss German offers both the best example for diglossia (all speakers are native speakers of Swiss German and thus diglossic) and the worst, because there is no clear-cut hierarchy.
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