linguistic:paraphrasing, stylistic:style_transfer
Language registers and types of diglossia

In his 1959 article, Charles A. Ferguson defines diglossia as follows:

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.

Here, diglossia is seen as a kind of bilingualism in a society in which one of the languages has high prestige (henceforth referred to as "H"), and another of the languages has low prestige ("L"). In Ferguson's definition, the high and low variants are always closely related.

Ferguson gives the example of standardized Arabic and says that, "very often, educated Arabs will maintain they never use L at all, in spite of the fact that direct observation shows that they use it constantly in ordinary conversation"

Joshua Fishman expanded the definition of diglossia to include the use of unrelated languages as high and low varieties. For example, in Alsace the Alsatian language (Elsässisch) serves as (L) and French as (H). Heinz Kloss calls the (H) variant exoglossia and the (L) variant endoglossia.

In some cases (especially with creole languages), the nature of the connection between (H) and (L) is not one of diglossia but a continuum; for example, Jamaican Creole as (L) and Standard English as (H) in Jamaica. Similar is the case in the Scottish Lowlands, with Scots as (L) and Scottish English as (H).

(H) is usually the written language whereas (L) is the spoken language. In formal situations, (H) is used; in informal situations, (L) is used. Sometimes, (H) is used in informal situations and as spoken language when speakers of 2 different (L) languages and dialects or more communicate with each other (as a lingua franca), but not the other way around.

One of the earliest examples was that of Middle Egyptian, the language in everyday use in Ancient Egypt during the Middle Kingdom (2000–1650 BC). By 1350 BC, in the New Kingdom (1550–1050 BC), the Egyptian language had evolved into Late Egyptian, which itself later evolved into Demotic (700 BC – AD 400). These two later forms served as (L) languages in their respective periods. But in both cases, Middle Egyptian remained the standard written, prestigious form, the (H) language, and was still used for this purpose until the fourth century AD, more than sixteen centuries after it had ceased to exist in everyday speech.

Another historical example is Latin, Classical Latin being the (H) and Vulgar Latin the (L); the latter, which is almost completely unattested in text, is the tongue from which the Romance languages descended.

The (L) variants are not just simplifications or "corruptions" of the (H) variants. In phonology, for example, (L) dialects are as likely to have phonemes absent from the (H) as vice versa. Some Swiss German dialects have three phonemes, /e/, /ɛ/ and /æ/, in the phonetic space where Standard German has only two phonemes, /ɛ(ː)/ (Berlin 'Berlin', Bären 'bears') and /eː/ (Beeren 'berries'). Jamaican Creole has fewer vowel phonemes than standard English, but it has additional palatal /kʲ/ and /ɡʲ/ phonemes.

Especially in endoglossia the (L) form may also be called " basilect ", the (H) form " acrolect ", and an intermediate form " mesolect ".

Ferguson's classic examples include Standard German/Swiss German, Standard Arabic / Arabic vernaculars, French / Creole in Haiti, and Katharevousa / Dimotiki in Greece, though the "low prestige" nature of most of these examples has changed since Ferguson's article was published. Creole is now recognized as a standard language in Haiti; Swiss German dialects are hardly low-prestige languages in Switzerland (see Chambers, Sociolinguistic Theory); and after the end of the Greek military regime in 1974, Dimotiki was made into Greece's sole standard language in 1976, and nowadays, Katharevousa is (with a few exceptions) no longer used. Harold Schiffman wrote about Swiss German in 2010, "It seems to be the case that Swiss German was once consensually agreed to be in a diglossic hierarchy with Standard German, but that this consensus is now breaking." Code-switching is also commonplace, 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.

Examples where the High/Low dichotomy is justified in terms of social prestige include Italian dialects and languages 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 non-standard dialects typically use those dialects 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 of diglossia (all speakers are native speakers of Swiss German and thus diglossic) and the worst, because there is no clear-cut hierarchy. While Swiss Standard German is spoken in formal situations like in school, news broadcasts, and government speeches, Swiss Standard German is also spoken in informal situations only whenever a German Swiss is communicating with a German-speaking foreigner who it is assumed would not understand the respective dialect. Amongst themselves, the German-speaking Swiss use their respective Swiss German dialect, irrespective of social class, education or topic.

In most African countries, as well as some Asian ones, a European language serves as the official, prestige language, and local languages are used in everyday life outside formal situations. For example, Wolof is the everyday lingua franca in Senegal, French being spoken only in very formal situations, and English is spoken in formal situations in Nigeria, but native languages like Hausa, Igbo, and Yoruba are spoken in ordinary conversation. However, a European language that serves as an official language can also act as a lingua franca, being spoken in informal situations between speakers of two or more different languages to facilitate communication. Diglossia can exist between two dialects of a European language as well. For example, in Côte d'Ivoire, Standard French is the prestige language used in business, politics, etc. while Ivorian French is the daily language in the street, on the markets, and in informal situations in general; in Mozambique, European Portuguese is used in formal situations, while Mozambican Portuguese is the spoken language in informal situations; and British English is used in formal situations in Nigeria, while Nigerian English is the spoken language in informal situations. In the countryside, local African dialects prevail. However, in traditional events, local languages can be used as prestige dialects: for example, a wedding ceremony between two young urban Baoulé people with poor knowledge of the Baoulé language (spoken in Côte d'Ivoire) would require the presence of elder family members as interpreters to conduct the ceremony in that language. Local languages, if used as prestige languages, are also used in writing materials in a more formal type of vocabulary. There are also European languages in Africa, particularly North Africa, without official status that are used as prestige language: for example, in Morocco, while Modern Standard Arabic and recently Tamazight are the only two official languages used in formal situations, with Moroccan Arabic and Amazigh dialects spoken in informal situations, while French and Spanish are also spoken in formal situations, making some Moroccans bilinguals or trilinguals in Modern Standard Arabic or Tamazight, French or Spanish, and Moroccan Arabic or Amazigh dialects. In Sahrawi Arab Democratic Republic, the official languages are Modern Standard Arabic and Spanish, which are spoken in formal situations, while Hassaniya Arabic is spoken in informal situations, and Spanish is also spoken in informal situations. In Asia, the Philippines is the biggest example of such colonial exoglossia, with English since the Spanish-American War of 1898, Spanish before then (with a historic presence in place names, personal names, and loanwords in the local languages) and local Austronesian Philippine languages used for everyday situations; Timor-Leste is in a similar situation with Portuguese. Most Asian countries instead have re-established a local prestige language (such as Hindi or Indonesian) and have at least partially phased out the colonial language, commonly English or Russian but also Dutch, French, and Portuguese in a few places, except for international, business, scientific, or interethnic communication; the colonial languages have also usually left many loanwards in the local languages.