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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 intentional study or special effort. It is usually used as the most 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 understand of 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.
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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/or practical application. 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. An example of a pair of genetically related languages is a creole and a parent language.
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 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 
There is no formal distinction between two distinct languages and two varieties of a single language, but linguists generally use mutual intelligibility as one of the primary factors in deciding between the two cases. Some linguists[who?] claim that mutual intelligibility is, ideally at least, the primary criterion separating languages from dialects. On the other hand, speakers of closely related languages can often communicate with each other to a fair degree; thus there are varying degrees of mutual intelligibility, and often other criteria are also used. As an example, in the case of a linear dialect chain that shades gradually between varieties, where speakers near the center can understand the varieties at both ends, but speakers at one end cannot understand the speakers at the other end, the entire chain is often considered a single language. If the central varieties then 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.
In addition, political and social conventions often override considerations of mutual intelligibility. For example, the varieties of Chinese and of Arabic are often each considered a single language even though there is often no mutual intelligibility between geographically separated varieties. 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. To deal with the conflict in cases such as Arabic, Chinese, and German, the term Dachsprache (a sociolinguistic 'umbrella' language) is sometimes seen: Arabic, 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 
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 not vice versa. For example, Dutch speakers tend to find it much easier to understand Afrikaans than vice versa as a result of Afrikaans's simplified grammar.
In other cases, two languages have very similar written forms, but are pronounced very differently. If the spoken form of one of the languages is more similar to the common written form, speakers of the other language may understand this language more than vice versa. This may account for the common claim that Portuguese speakers can understand Spanish more easily than the other way around, since certain letters that are largely written the same in both languages (e.g. ‹a e i o u r n s›) tend to have only one pronunciation in Spanish (or if there are multiple pronunciations, they are similar) but they have multiple, often very different pronunciations in Portuguese depending on context and the position in a word.
However, perhaps the most common reason for apparent asymmetric intelligibility is that speakers of one variety have more exposure to the other than vice versa. For example, speakers of Scottish English have frequent exposure to American English through movies and TV programs, whereas speakers of American English have little exposure to Scottish English; hence, American English speakers often find it difficult understanding Scottish English or, especially, Scots (not formal Scottish Standard English), whereas Scots tend to have few problems understanding American English. Danish and Swedish normally have low mutual intelligibility, but Swedes in Malmö, across a strait from the Danish capital Copenhagen, understand Danish somewhat better (see North Germanic languages, "Mutual intelligibility").
In some cases it is hard to distinguish between mutual intelligibility and a basic knowledge of other language. Many Belarusian and Ukrainian speakers have extensive knowledge of Russian and use it as a second language or lingua franca, or even as a first language in public or at work. Thus they can easily understand Russian, whereas speakers of Russian often can understand Ukrainian and Belarusian only partially. Similarly, in Germany and Italy, standard German or Italian speakers have great difficulty understanding the 'dialects' from regions other than their own, but virtually all 'dialect' speakers learn the standard languages in school and from the media.
Norwegian Bokmål and Standard Danish are asymmetrically intelligible. Speakers of Norwegian can understand Danish better than vice versa. The reason for this may be that Norwegians are more used to listening to speakers of different dialectal backgrounds.
List of mutually intelligible languages 
Written and spoken forms 
- Afrikaans: Dutch (partially)
- Azerbaijani: Turkish (both partially and asymmetrically)
- Belarusian: Russian (partially) and Ukrainian (partially)
- Bulgarian: Macedonian
- Czech: Slovak
- Danish: Norwegian and Swedish (both partially)
- Dutch: Afrikaans (partially), West Frisian (partially), and German (partially)
- English: Scots (partially) [better source needed]
- Estonian: Finnish (partially)
- Finnish: Estonian (partially), Karelian (partially)
- Galician: Portuguese
- German: Dutch (partially)
- Ilokano language: Bontoc language (partially) and Balangao language (partially) 
- Irish: Scottish Gaelic (partially)
- Kazakh: Kyrgyz
- Kinyarwanda: Kirundi
- Kirundi: Kinyarwanda
- Kyrgyz: Kazakh
- Macedonian: Bulgarian, Serbo-Croatian (partially and asymmetrically)
- Norwegian: Danish (partially) and Swedish
- Portuguese: Galician and Spanish (partially)
- Russian: Belarusian and Ukrainian (both partially)
- Scots: English (partially) [better source needed]
- Slovak: Czech
- Slovenian: Serbo-Croatian (partially)
- Spanish: Galician and Portuguese (partially)
- Swedish: Danish (partially) and Norwegian
- Thai: Lao
- Tokelauan: Tuvaluan
- Torlakian (when written in the Cyrillic alphabet): Bulgarian, Macedonian and Serbo-Croatian (when written in the Cyrillic alphabet)
- Turkish: Azerbaijani (both partially and asymmetrically)
- Tuvaluan: Tokelauan
- Ukrainian: Belarusian and Russian (both partially)
Spoken forms only 
- Dari: Tajik (because Tajik is currently written in Cyrillic alphabet and Dari in Perso-Arabic script).
- German: Yiddish (because German is written in Latin script and Yiddish in Hebrew script)
- Lao: Thai (because Lao is written in Lao alphabet and Thai in Thai alphabet)
- Persian: Tajik (because Persian is written in Perso-Arabic script and Tajik is currently written in Cyrillic script)
- Tajik: Persian and Dari (because Tajik is currently written in Cyrillic and Persian and Dari in Perso-Arabic script).
- Thai: Lao
- Uyghur: Uzbek (because Uyghur is usually written in Uyghur Ereb Yëziqi Arabic script and Uzbek in the Latin script)
- Uzbek: Uyghur
- Yiddish: German
Written forms only 
In ancient times 
Dialects or registers of one language sometimes considered separate languages 
- Hindustani: Hindi, 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
- Malay: Indonesian, Malaysian
- Serbo-Croatian: Bosnian, Croatian, Montenegrin, and Serbian—the standard forms are structurally the same language, and hence mutually intelligible, spoken and written (if the Latin alphabet is used). They are considered separate languages only for political reasons.
- Tagalog: Filipino—the national language of the Philippines, Filipino, is based almost entirely on the Luzon dialects of Tagalog.
See also 
- Dialect continuum
- Dialect levelling
- Language secessionism
- Lexical similarity
- Non-convergent discourse
- Pluricentric language
- Standard language
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