Asymmetric intelligibility is a term used by linguists to describe two languages that are considered mutually intelligible, but where one group of speakers has more difficulty understanding the other language than vice versa. 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.
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.
Similarly, Dutch speakers tend to find it much easier to understand Afrikaans than vice versa as a result of Afrikaans's simplified grammar.
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, while speakers of American English have little exposure to Scottish English; hence, American English speakers often find it difficult to understand Scottish English, while Scottish English speakers tend to have few problems understanding American English.
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. Thus they can easily understand Russian, while speakers of Russian often can understand Ukrainian and Belarusian only partially.
Norwegian Bokmål and Standard Danish have some asymmetric intelligibility, as speakers of Norwegian can understand Danish better than vice versa. The reason for this is uncertain.
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