In the field of dialectology, a diasystem or polylectal grammar[a] is a linguistic analysis set up to encode or represent a range of related[b] varieties in a way that displays their structural differences.
Inspired by Trubetzkoy, Weinreich (1954) proposed a synthesis of linguistic geography and descriptive linguistics by applying the structuralist concept of grammar to the description of regular correspondences between different varieties;[15] a resulting supergrammar, which he called a diasystem, would be consistent with the individual grammars of all the member dialects.
Weinreich exemplified the diasystemic approach by a formulaic arrangement of phoneme correspondences in three dialects of Yiddish, focusing on the vowels but arguing that the principle could work for other aspects of language.
Moulton (1960) found an extreme example of divergent incidence in a study of two dialects of Swiss German, Luzerner and Appenzeller, which evolved independently of each other.
The remaining phonetic similarities between the Luzerner and Appenzeller phoneme sets were fortuitous results of multiple mergers and splits that each dialect underwent separately.
[20] Pulgram (1964), examining Cochrane (1959) and Moulton (1960), noted the need for refinements in the original proposal; different researchers did not seem to agree on definitions, disciplines of study, or objects of inquiry.
Related to Weinreich's proposal were efforts in both American dialectology and generative phonology to construct an "overall system" that represented the underlying representation for all dialects of English.
[27] Six of the nine simple vowels in this diasystem are common across most dialects: /i/ occurs in pit, /e/ in pet, /æ/ in pat, /ə/ in putt, /u/ in put, and /a/ in pot.
[35] Still, these sorts of "idiosyncratic" grammars differ in degree from broader diasystems, which are much less likely to be part of speakers' linguistic competence.