fit. Six of the nine simple vowels in this diasystem are common across most dialects: occurs in pit, in pet, in pat, in putt, in put, and in pot. The other three are found in specific dialects or dialect groups: represents the vowel of road in New England varieties; represents a vowel that often appears in stressed syllables in words like just (when it means 'only'); and represents the vowel of pot in Southern British and New England dialects. These nine simple vowels can then be combined with any of three offglides () to make 36 possible complex nuclei. This system was popular amongst American linguists (despite criticism, particularly from Hans Kurath) until demonstrated its inadequacy. The most salient criticism of these broad diasystems was the issue of how cognitively real they are. That is, whether speakers actually have competence in using or understanding the grammatical nuances of multiple varieties. In certain sociolinguistic circumstances, speakers' linguistic repertoire contains multiple varieties. For example, argues that Modern Literary Arabic is a diasystem of various interference phenomena occurring when speakers of different Arabic varieties attempt to speak or read Literary Arabic. More concretely, Peter Trudgill put forth what he considered to be a cognitively real diasystem in , a book-length sociolinguistic study of Norwich. As a critic of Weinreich's original proposal, he approached the concept as a generativist, putting forth a series of rules that could generate any possible output reflected in the diversity and variability of sociologically conditioned linguistic variables. Because most speakers of Norwich could vary their pronunciation of each variable depending on the circumstances in which they are speaking, the diasystem's rules reflected speakers' actual linguistic abilities. Cognitively real diasystems are not limited to humans. For example, crows are able to distinguish between different calls that prompt others to disperse, assemble, or rescue; these calls show regional variation so that French crows do not understand recorded American calls. Although captive birds show difficulty understanding the calls of birds from nearby regions, those allowed to migrate are able to understand calls from both, suggesting that they have mentally constructed a diasystem that enables them to understand both call systems. Still, these sorts of "idiosyncratic" grammars differ in degree from broader diasystems, which are much less likely to be part of speakers' linguistic competence. Even Trudgill has argued against their cognitive reality, deeming the concept of a broad diasystem to be a