Hapur Junction railway station in the context of "Dehradun"

In linguistics, diglossia (/daɪˈɡlɒsiə/ dy-GLOSS-ee-ə, US also /daɪˈɡlɔːsiə/ dy-GLAW-see-ə) is where two dialects or languages are used (in fairly strict compartmentalization) by a single language community. In addition to the community's everyday or vernacular language variety (labeled "L" or "low" variety), a second, highly codified lect (labeled "H" or "high") is used in certain situations such as literature, formal education, or other specific settings, but not used normally for ordinary conversation. The H variety may have no native speakers within the community. In cases of three dialects, the term triglossia is used. When referring to two writing systems coexisting for a single language, the term digraphia is used.

The high variety may be an older stage of the same language (as in medieval Europe, where Latin (H) remained in formal use even as colloquial speech (L) diverged), an unrelated language, or a distinct yet closely related present-day dialect (as in northern India and Pakistan, where Hindustani (L) is used alongside the standard registers of Hindi (H) and Urdu (H); Germany, where Hochdeutsch (H) is used alongside German dialects (L); the Arab world, where Modern Standard Arabic (H) is used alongside other varieties of Arabic (L); and China, where Standard Chinese (H) is used as the official, literary standard and local varieties of Chinese (L) are used in everyday communication); in Dravidian languages, Tamil has the largest diglossia with Literary Tamil (H) used in formal settings and colloquial spoken Tamil (L) used in daily life. Other examples include literary Katharevousa (H) versus spoken Demotic Greek (L); Indonesian, with its bahasa baku (H) and bahasa gaul (L) forms; Standard American English (H) versus African-American Vernacular English or Hawaiian Pidgin (L); and literary (H) versus spoken (L) Welsh.

The Hindi–Urdu controversy was a dispute that arose in 19th-century British India over whether Modern Standard Hindi or Standard Urdu should be chosen as a national language. It is considered one of the leading Hindu–Muslim issues of British India.

Hindi and Urdu are mutually intelligible standard registers of the Hindustani language (also known as Hindi–Urdu). The respective writing systems used to write the language, however, are different: Hindi is written in the Devanagari variant of the Brahmic scripts whereas Urdu is written using a modified Nastaliq variant of the Arabic script, each of which is completely unintelligible to readers literate only in one or the other. Both Modern Standard Hindi and Urdu are literary forms of the Dehlavi dialect of Hindustani. A Persianised variant of Hindustani began to take shape during the Delhi Sultanate (1206–1526) and Mughal Empire (1526–1858) in South Asia. Known as Deccani in South India, and by names such as Hindi, Hindavi, and Hindustani in North India and elsewhere, it emerged as a lingua franca across much of Northern India and was written in several scripts including Devanagari, Perso-Arabic, Kaithi, and Gurmukhi.