In discussing contextual cues to vowel identifications in English, note that controlling for dialect is largely unimportant for eliciting identifications when vowels are placed between consonants, possibly because the /CVC/ structure often forms lexical items that can aid in identification; identifying vowels in isolation, which don't often carry such lexical information, must be matched to the listener's set of vowel prototypes with less deviation than in consonantal contexts. In the first chapter of , Peter Trudgill makes the case that these semantic contexts form the basis of intelligibility across varieties and that the process is irregular and "ad hoc" rather than the result of any sort of rule-governed passive polylectal competence.