In linguistics, in the field of structural dialectology, a diasystem is a single genetic language which has two or more standard forms. Some dialects are often divided into separate languages due to different historical and cultural development. Other possible differences between languages include vocabulary, such as Occitan being affected by French and Catalan by Spanish words, and writing systems, such as Hindi in Devanagari and Urdu in the Arabic script, despite being mutually intelligible. Some languages are officially recognized as distinct despite having no barriers in speech, writing or lexicon, but are distinguished by legal and political factors, such as the Catalan with Valencian, and Romanian with Moldovan. Examples include:

Bulgarian-Macedonian-Torlakian
Czech-Slovak (Czech-Slovak-Pannonian Rusyn)
Danish-Bokmål Norwegian-Nynorsk Norwegian
Filipino-Tagalog
Hindustani (Hindi-Urdu)
Irish-Scottish Gaelic
Lao-Isan
Malay (Malaysian-Indonesian)
Mandarin Chinese-Dungan
Occitano-Romance languages (Occitan-Catalan-Valencian)
Persian-Tajik-Dari
Portuguese-Galician (Portuguese-Galician)
Romanian-Moldovan
Central South Slavic (Bosnian-Croatian-Montenegrin-Serbian)
Spanish-Ladino
Tatar-Bashkir
Tuscan (Italian-Corsican)
Ukrainian-Rusyn
Uyghur-Uzbek