result of Afrikaans' simplified grammar. Among sign languages Sign languages are not universal and are usually not mutually intelligible, although there are also similarities among different sign languages. Sign languages are independent of spoken languages and follow their own paths of development. For example, British Sign Language (BSL) and American Sign Language (ASL) are quite different and mutually unintelligible, even though the hearing people of the United Kingdom and the United States share the same spoken language. The grammars of sign languages do not usually resemble those of spoken languages used in the same geographical area; in fact, in terms of syntax, ASL shares more with spoken Japanese than it does with English. As a criterion for identifying separate languages 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. 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. 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