Vowels that appear in one form of a word may be elided in speech (and even in writing) depending on the context, especially between consonants but not only. This elision (called “syncope”) occurs in many languages of the world including European languages like English or Russian and is typically the product of a historical process, wherein earlier “full vowel” forms (i. e., unsyncopated) alternate with newer forms that omit the vowel in question. The occurrence of syncope is routinely associated with the stylistic dimension of language, specifically with the so-called “elliptic code,” and contrasted with the “explicit code” wherein the vowel in question appears unelided. (These terms were introduced for the first time into the discourse of linguistics in Y-H-B’s first book, Russian Phonetic Variants and Phonostylistics [University of California Press, 1968]). The elliptic form tends to be generalized over time at the expense of the explicit one, as often happens under the appropriate circumstances in the pronunciation of nomina propria, including English (British) place names such as Leicester (pronounced [‘lɛstər]); cf. the colloquial syncopated pronunciation of the British English word governor as [‘ɡʌvnə(r)], occasionally rendered as guvna orthographically to reflect the colloquial phonetics.
In English the archetypical instance of syncope is in contractions. Thus, for example, the subject-verb combination “I am” is characteristic of the explicit code but is reduced to “I’m” (where the apostrophe marks contraction) in the elliptic code.
Certain phonetic contexts are more likely to induce syncope than others. The occurrence of a vowel in an unstressed syllable is a sine qua non by itself. From that basic starting point, the occurrence of an adjacent sonorant in the syllable––more specifically, a nasal consonant like /n/ or /m/––often leads to the unstressed vowel being dropped, as when heaven is pronounced (esp. in British English) in the second syllable without the vowel and a syllabic nasal.
Traditional phonetic explanations of vowel syncope rely on such notions as economy of effort, but this is clearly inadequate, even though items in the elliptic code tend to be pronounced faster than their counterparts in the explicit code. The function of vowel syncope is rather the usual semiotic one, viz. of mapping the hierarchy of distinctive features that define a phoneme through its instantiation contextually in speech. Thus vowels––which are defined as [+ vocalic] and [- consonantal]––signify this definition in connected speech by being liable to syncope, the only speech sounds which function that way in the rules of implementation characterizing a phonological system.