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.
A 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 officially 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.
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 between Karakalpak, Kazakh and Uzbek languages is currently under way at the University of Surrey.
Below is an incomplete list of fully and partially mutually intelligible varieties sometimes considered languages.Dialects or registers of one language sometimes considered separate languagesThe Linguasphere register of the world’s languages and speech communities