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.
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.
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.
Similarly, in the German speaking area and Italy, standard German or Italian speakers may have great difficulty understanding the vernaculars from regions other than their own, but virtually all speakers of non-standard varieties learn the standard dialects in school and from the media.
Below is an incomplete list of fully and partially mutually intelligible varieties sometimes considered languages.
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:
lexical differences between the ethnic variants are extremely limited, even when compared with those between closely related Slavic languages (such as standard Czech and Slovak, Bulgarian and Macedonian), and grammatical differences are even less pronounced. More importantly, complete understanding between the ethnic variants of the standard language makes translation and second language teaching impossible