In linguistics, mutual intelligibility is a relationship between languages or dialects in which speakers of different but related varieties can readily understand each other without prior familiarity or special effort. It is sometimes used as an important criterion for distinguishing languages from dialects, although sociolinguistic factors are often also used.
Intelligibility between languages can be asymmetric, with speakers of one understanding more of the other than speakers of the other understanding the first. When it is relatively symmetric, it is characterized as "mutual". It exists in differing degrees among many related or geographically proximate languages of the world, often in the context of a dialect continuum.
Linguistic distance is the name for the concept of calculating a measurement for how different languages are from one another. The higher the linguistic distance, the lower the mutual intelligibility.
For individuals to achieve moderate proficiency or understanding in a language (called L2) other than their first language (L1) typically requires considerable time and effort through study and practical application. Advanced speakers of a second language typically aim for intelligibility, especially in situations where they work in their second language and the necessity of being understood is high. However, many groups of languages are partly mutually intelligible, i.e. most speakers of one language find it relatively easy to achieve some degree of understanding in the related language(s). Often the languages are genetically related, and they are likely to be similar to each other in grammar, vocabulary, pronunciation, or other features.
Intelligibility among languages can vary between individuals or groups within a language population according to their knowledge of various registers and vocabulary in their own language, their exposure to additional related languages, their interest in or familiarity with other cultures, the domain of discussion, psycho-cognitive traits, the mode of language used (written vs. oral), and other factors.
Mutually intelligible languages or varieties of one language
Some linguists use mutual intelligibility as a primary criterion for determining whether two speech varieties represent the same or different languages. In a similar vein, some claim that mutual intelligibility is, ideally at least, the primary criterion separating languages from dialects.
The primary challenge to these positions is that speakers of closely related languages can often communicate with each other effectively if they choose to do so. In the case of transparently cognate languages recognized as distinct such as Spanish and Italian, mutual intelligibility is in principle and in practice not binary (simply yes or no), but occurs in varying degrees, subject to numerous variables specific to individual speakers in the context of the communication.
Classifications may also shift for reasons external to the languages themselves. As an example, in the case of a linear dialect continuum that shades gradually between varieties, where speakers near the center can understand the varieties at both ends with relative ease, but speakers at one end have difficulty understanding the speakers at the other end, the entire chain is often considered a single language. If the central varieties die out and only the varieties at both ends survive, they may then be reclassified as two languages, even though no actual language change has occurred during the time of the loss of the central varieties. In this case, too, however, while mutual intelligibility between speakers of the distant remnant languages may be greatly constrained, it is likely not at the zero level of completely unrelated languages.
In addition, political and social conventions often override considerations of mutual intelligibility in both scientific and non-scientific views. For example, the varieties of Chinese are often considered a single language even though there is usually no mutual intelligibility between geographically separated varieties. Another similar example would be varieties of Arabic. In contrast, there is often significant intelligibility between different Scandinavian languages, but as each of them has its own standard form, they are classified as separate languages. There is also significant intelligibility between Thai languages of different regions of Thailand.
Many Turkic languages are also mutually intelligible to a higher or lower degree, but thorough empirical research is needed to establish the exact levels and patterns of mutual intelligibility between the languages of this linguistic family. The British Academy funded research project dedicated to examining mutual intelligibility between Karakalpak, Kazakh and Uzbek languages is currently under way at the University of Surrey.
To deal with the conflict in cases such as Arabic, Chinese and German, the term Dachsprache (a sociolinguistic "umbrella language") is sometimes seen: Chinese and German are languages in the sociolinguistic sense even though some speakers cannot understand each other without recourse to a standard or prestige form.
Asymmetric intelligibility refers to two languages that are considered partially mutually intelligible, but where one group of speakers has more difficulty understanding the other language than the other way around. There can be various reasons for this. If, for example, one language is related to another but has simplified its grammar, the speakers of the original language may understand the simplified language, but less vice versa. For example, Dutch speakers tend to find it easier to understand Afrikaans than vice versa as a result of Afrikaans' simplified grammar.
Northern Germanic languages spoken in Scandinavia form a dialect continuum where two furthermost dialects have almost no mutual intelligibility. As such, spoken Danish and Swedish normally have low mutual intelligibility, but Swedes in the Öresund region (including Malmö and Helsingborg), across a strait from the Danish capital Copenhagen, understand Danish somewhat better, largely due to the proximity of the region to Danish-speaking areas. While Norway was under Danish rule, the Bokmål written standard of Norwegian developed from Dano-Norwegian, a koiné language that evolved among the urban elite in Norwegian cities during the later years of the union. Additionally, Norwegian assimilated a considerable amount of Danish vocabulary as well as traditional Danish expressions. As a consequence, spoken mutual intelligibility is not reciprocal.
List of mutually intelligible languages
Written and spoken forms
- Afrikaans: Dutch (partially)
- Azerbaijani: Crimean Tatar, Gagauz, Turkish and Urum (partially and asymmetrically)[verification needed]
- Belarusian: Russian and Ukrainian (both partially)
- Bulgarian: Macedonian (significantly) 
- Cebuano: Hiligaynon (significantly)
- Crimean Tatar: Azerbaijani, Gagauz, Turkish and Urum (partially and asymmetrically)[verification needed]
- Czech: Slovak (significantly), Polish (partially)
- Danish: Norwegian and Swedish (both partially and asymmetrically)
- Dutch: Afrikaans (in written form; in spoken form partially), Limburgish and West Frisian (partially)
- English: Scots (significantly)
- Estonian: Finnish (partially)
- Finnish: Estonian (partially), Karelian (significantly) Kven and Meänkieli (significantly)
- Gagauz: Azerbaijani, Crimean Tatar, Turkish and Urum (partially and asymmetrically)[verification needed]
- German: Luxembourgish (partially)
- Hiligaynon: Capiznon (significantly) and Cebuano (significantly)
- Irish: Scottish Gaelic (partially; varies greatly according to dialect. The greatest mutual intelligibility is between Ulster Irish and southern Scottish dialects.). See also: Comparison of Scottish Gaelic and Irish.
- Italian: Corsican (significantly), Spanish, Portuguese and Catalan (partially)
- Limburgish: Dutch and Afrikaans (partially)
- Luxembourgish: German (partially)
- Macedonian: Bulgarian (significantly), Serbo-Croatian (partially and asymmetrically)
- Maltese: Tunisian Arabic (significantly) and Sicilian (partially)
- Manchu: Xibe
- Marathi: Konkani (significantly) 
- Moroccan Arabic: Algerian Arabic (significantly), yet the mutual intelligibility degree may vary depending on local dialects
- Norwegian: Danish and Swedish (both partially and asymmetrically)
- Polish: Slovak (reasonably), Czech (partially)
- Portuguese: Galician (significantly), Spanish (significantly in written form; asymmetrically in spoken form), Catalan (partially) and Italian (partially)
- Russian: Belarusian and Ukrainian (both partially)
- Serbo-Croatian: Slovene (partially and asymmetrically), Macedonian (partially and asymmetrically)
- Slovak: Czech (significantly), Polish (reasonably)
- Slovene: Serbo-Croatian (partially and asymmetrically)
- Spanish: Portuguese (significantly in written form; asymmetrically in spoken form), Italian (partially) and Catalan (partially)
- Swedish: Danish and Norwegian (both partially and asymmetrically)
- Tunisian Arabic: Maltese (significantly), Algerian Arabic and Libyan Arabic (both partially)
- Turkish: Azerbaijani, Crimean Tatar, Gagauz and Urum (partially and asymmetrically)[verification needed]
- Ukrainian: Belarusian and Russian (both partially)
- Urum: Azerbaijani, Crimean Tatar, Gagauz and Turkish (partially and asymmetrically)[verification needed]
- Welsh: Breton and Cornish (both partially)
- Xibe: Manchu
- Zulu: Northern Ndebele (partially), Xhosa (partially), and Swazi (partially; the first three are often considered to be dialects of a uniform Zunda language)
Spoken forms mainly
- Hindi: Urdu. See the article on Hindustani language.
- Akha, Honi, Hani (variety of different written scripts)
- Dungan: Mandarin, especially with Central Plains Mandarin (partially; Dungan is usually written in Cyrillic and Mandarin usually in Chinese characters)
- German: Yiddish (because German is usually written in Latin script and Yiddish usually in the Hebrew alphabet). However, Yiddish's use of many borrowed words, chiefly from Hebrew and Slavic languages, makes it more difficult for a German speaker to understand spoken Yiddish than the reverse.
- Polish: Ukrainian and Belarusian (both partially; moreover, Belarusian and Ukrainian are written in Cyrillic, while Polish is written in Latin)
- Spanish: Judeo-Spanish (because Spanish is usually written in Latin script and Judeo-Spanish usually in the Hebrew alphabet). Depending on dialect and the number of non-Spanish loanwords used.
- Thai: Lao, Isan, Southern Thai, Northern Thai, Shan and Tai Lue (both partially and asymmetrically, with every language having its own script except that Thai and Southern Thai use the same script.)
Written forms mainly
- French: Italian, Portuguese and Spanish. French in its spoken form is not mutually intelligible with Spanish, Portuguese, or Italian due to the great phonological changes that it underwent in recent centuries, but it has partial intelligibility with all three in written form because it preserves its writing from Middle French with very few changes. According to phonological studies, French is the Romance language that has distanced itself the most from Latin. Also, the use of some Germanic words in the common lexicon makes it more difficult for speakers of other Romance languages to understand. According to Ethnologue, French has 89% lexical similarity with Italian and 75% with Portuguese and Spanish (both of which have incorporated some Arabic vocabulary).
- German: Dutch. Standard Dutch and Standard German show a limited degree of mutual intelligibility when written. One study concluded that when concerning written language, Dutch speakers could translate 50.2% of the provided German words correctly, while the German test subjects were able to translate 41.9% of the Dutch equivalents correctly. Another study showed that while Dutch speakers could correctly translate 71% of German cognates, they could only translate 26.6% of non-cognates correctly, suggesting a widely fluctuating intelligibility. In terms of orthography, 22% of the vocabulary of Dutch and German is identical or near identical. The Levenshtein distance between written Dutch and German is 50.4% as opposed to 61.7% between English and Dutch. The spoken languages are much more difficult to understand for both. Studies show Dutch speakers have slightly less difficulty in understanding German speakers than vice versa. It remains unclear whether this asymmetry has to do with prior knowledge of the language (Dutch people are more exposed to German than vice versa), better knowledge of another related language (English) or any other non-linguistic reasons.
- Icelandic: Faroese.
- Chechen: Ingush.
List of mutually intelligible varieties
Below is an incomplete list of fully and partially mutually intelligible varieties sometimes considered languages.
- Dari: Persian and Tajik
- Karakalpak: Kazakh and Nogai
- Kazakh: Karakalpak, Nogai, Altay and Kyrgyz
- Kinyarwanda: Kirundi
- Kirundi: Kinyarwanda
- Kyrgyz: Kazakh and Altay and Karakalpak
- Persian: Dari and Tajik
- Samoan: Tokelauan and Tuvaluan (partially)
- Tajik: Dari and Persian
- Tokelauan: Tuvaluan and Samoan (partially)
- Tuvaluan: Tokelauan and Samoan (partially)
Dialects or registers of one language sometimes considered separate languages
- Akan: Twi and Fante.
- Northeastern Neo-Aramaic (NENA) is a dialect continuum, with some dialects being mutually intelligible and others not. While Zakho Jewish Neo-Aramaic and Zakho Christian Neo-Aramaic are mutually intelligible, especially on the eastern edge (in Iran), Jewish and Christian NENA varieties spoken in the same town are not mutually intelligible.
- Catalan: Valencian – the standard forms are structurally the same language and share the vast majority of their vocabulary, and hence highly mutually intelligible. They are considered separate languages only for political reasons.
- Hindustani: Hindi and Urdu – the standard forms are separate registers of structurally the same language (called Hindustani or Hindi-Urdu), with Hindi written in Devanagari and Urdu mainly in a Perso-Arabic script, and with Hindi drawing its literary and formal vocabulary mainly from Sanskrit and Urdu drawing it mainly from Persian and Arabic.
- Malay: Indonesian (the standard regulated by Indonesia) and Malaysian (the standard used in Malaysia, Brunei and Singapore). Both varieties are based on the same material basis and hence are generally mutually intelligible, despite the numerous lexical differences. Certain linguistic sources also treat the two standards on equal standing as varieties of the same Malay language. Malaysians tend to assert that Malaysian and Indonesian are merely different normative varieties of the same language, while Indonesians tend to treat them as separate, albeit closely related, languages. However, vernacular or less formal varieties spoken between these two countries share limited intelligibility, evidenced by Malaysians having difficulties understanding Indonesian sinetron (soap opera) aired on their TV stations, and vice versa.
- Serbo-Croatian: Bosnian, Croatian, Montenegrin, and Serbian – the national varieties are structurally the same language, all constituting normative varieties of the Shtokavian dialect, and hence mutually intelligible, spoken and written (if the Latin alphabet is used). For political reasons, they are sometimes considered distinct languages.
- The non-standard vernacular dialects of Serbo-Croatian (Kajkavian, Chakavian and Torlakian) diverge more significantly from all four normative varieties. As Shtokavian has its own set of subdialects, they are considered by some linguists as separate languages, albeit closely related to Shtokavian Serbo-Croatian, rather than as Serbo-Croatian dialects. Their mutual intelligibility varies greatly, between the dialects themselves, with Shtokavian, and with other languages. Kajkavian has higher mutual intelligibility with Slovene than with the national varieties of Shtokavian, while Chakavian has a low mutual intelligibility with either, in part due to its large number of loanwords from Venetian. Torlakian (considered a subdialect of Serbian Old Shtokavian by some) has significant mutual intelligibility with Macedonian and Bulgarian. All South Slavic languages in effect form a large dialect continuum of gradually mutually intelligible varieties depending on distance between the areas where they are spoken.
- Romanian: "Moldovan" – the standard forms are structurally the same language, and hence mutually intelligible. They are considered separate languages only for political reasons and many Moldovans declare their language to be Romanian and their desire to unite with Romania.
- Tagalog: Filipino – the national language of the Philippines, Filipino, is based almost entirely on the Luzon dialects of Tagalog.
Because of the difficulty of imposing boundaries on a continuum, various counts of the Romance languages are given; in The Linguasphere register of the world’s languages and speech communities David Dalby lists 23 based on mutual intelligibility:
- Iberian Romance: Portuguese, Galician, Mirandese, Astur-Leonese, Spanish, Aragonese;
- Occitano-Romance: Catalan, Occitan;
- Gallo-Romance: Langues d'oïl (including French), Franco-Provençal;
- Rhaeto-Romance: Romansh, Ladin, Friulian;
- Gallo-Italic: Piedmontese, Ligurian, Lombard, Emilian-Romagnol, Venetian;
- Italo-Dalmatian: Corsican, Italian, Neapolitan, Sicilian, Istriot, Dalmatian (extinct);
- Eastern Romance: Daco-Romanian, Istro-Romanian, Aromanian, Megleno-Romanian.
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