There is no universally accepted criterion for distinguishing two different languages from two dialects (i.e., varieties) of the same language.
[4] A number of rough measures exist, sometimes leading to contradictory results. The distinction is therefore subjective and depends on the user's frame of reference.
The most common, and most purely linguistic, criterion is that of
mutual intelligibility: two varieties are said to be dialects of the same language if being a speaker of one variety confers sufficient knowledge to understand and be understood by a speaker of the other; otherwise, they are said to be different languages. However, this definition becomes problematic in the case of
dialect continua, in which it may be the case that dialect B is mutually intelligible with both dialect A and dialect C but dialects A and C are not mutually intelligible with each other. In this case the criterion of mutual intelligibility makes it impossible to decide whether A and C are dialects of the same language or not. Cases may also arise in which a speaker of dialect X can understand a speaker of dialect Y, but not vice versa; the mutual intelligibility criterion flounders here as well.
Another occasionally used criterion for discriminating dialects from languages is that of linguistic authority, a more
sociolinguistic notion. According to this definition, two varieties are considered dialects of the same language if (under at least some circumstances) they would defer to the same authority regarding some questions about their language. For instance, to learn the name of a new invention, or an obscure foreign species of plant, speakers of
Bavarian German and
East Franconian German might each consult a German dictionary or ask a German-speaking expert in the subject. By way of contrast, although Yiddish is classified by linguists as a language in the "Middle High German" group of languages, a Yiddish speaker would not consult a German dictionary to determine the word to use in such case.
By the definition most commonly used by linguists, any linguistic variety can be considered a "dialect" of
some language—"everybody speaks a dialect". According to that interpretation, the criteria above merely serve to distinguish whether two varieties are dialects of the
same language or dialects of
different languages.
A framework was developed in 1967 by
Heinz Kloss,
abstand and ausbau languages, to describe speech communities, that while unified politically and/or culturally, include multiple dialects which though closely related genetically may be divergent to the point of inter-dialect unintelligibility.