In linguistics, the term "mutual intelligibility" is a way to refer to one type of relationship between two languages. Languages are mutually intelligible if speakers of one can understand speakers of the other without significant difficulty or study. Mutual intelligibility is most commonly found among languages that are closely related to one another, but closely related languages are not always mutually intelligible.
There are several different kinds of mutual intelligibility. The term most commonly refers to mutual intelligibility between spoken and written languages. For instance, speakers of Czech can understand both spoken and written Slovak with little difficulty, while speakers of Slovak can also understand spoken and written Czech. Languages can be mutually intelligible to a greater or lesser degree. For instance, native Danish speakers can understand Norwegian and Swedish only partially, while native Norwegian speakers usually understand Danish only partially but Swedish to a much greater degree.
In some cases, mutual intelligibility is wholly or partly asymmetric. This means that it is easier for speakers of one language to understand the other than for speakers of the second language to understand the first. This is the case with Spanish and Portuguese; Portuguese speakers typically find it easier to understand Spanish than Spanish speakers find it to understand Portuguese. This difficulty arises largely from differences in pronunciation. The two languages are much easier for non-native speakers to understand in writing.
Some languages are mutually intelligible only in their spoken or written forms. For instance, since Yiddish derives from German, German speakers and Yiddish speakers can often understand one another. German is written in Latin characters, however, while Yiddish is written in Hebrew characters, meaning that the two languages are not mutually intelligible in their written forms. By contrast, Icelandic and Faroese are mutually intelligible primarily in their written forms, since written Faroese derives from Icelandic. Vast differences in pronunciation interfere with the mutual intelligibility of the spoken languages.
In some cases, mutual intelligibility can suggest that two languages are in fact dialects of the same language. This is the case in much of the former Yugoslavia, where different regions have distinct dialects of the Serb-Croatian language. These languages are almost totally mutually intelligible, but local desire for distinct ethnic identities results in their identification as different languages. In some cases, the degree of mutual intelligibility is impossible to ascertain, usually in the case of languages without living speakers. For example, historians and historical linguists debate the extent to which speakers of Old English and Old Norse, neither of which has a living speech community, could understand one another.