While vowel reduction may seem at first glance to be a single phenomenon, it is important to distinguish the many types that exist. Phonetic vowel reduction is not the same as phonological vowel reduction, and there are subtypes of each. While phonetic vowel reduction is considered to be a universal phonetic phenomenon, there are types of phonological vowel reduction that are quite different from language to language. We will discuss the types of phonological reduction in order to distinguish them from the subject of this study, phonetic vowel reduction.
One phonological type of reduction is reduction in vowel phoneme inventory in unstressed positions: fewer vowel qualities contrast there than in stressed positions. Exemplars range from Russian, which has one less vowel phoneme in unstressed positions, to most dialects of English, which are thought to have just one reduced vowel phoneme, //. Those Southern U.S. dialects discussed by Sledd (1966) are argued to have two reduction vowels, // and //; // may be considered the unstressed allophone of //. Bailey (1985:185) lists six unstressed vowels, [i, , , , , ], which may be considered as unstressed allophones of various stressed vowels. In all these analyses there is neutralization of contrasts in unstressed positions.
Another dimension of phonological vowel reduction is phonological mid centralization in unstressed positions. English is again at one extreme, where // is non-low and central, while Icelandic, with three peripheral ``reduced'' vowels /i,a,u/, is at the opposite extreme.
A third type of ``reduction'' is simply deletion of one out of a sequence of (phonologically identical) vowels, as discussed, for example, in Kenstowitz and Kisseberth (1979:128). This perhaps unrelated process will not be discussed further.
Finally, certain languages are said to lack vowel reduction (eg., Hindi and Spanish, cf. Dauer 1983), though the phonetic facts are relatively unknown (but see Delattre 1968). In summary, languages may or may not have either inventory reduction of vowel phonemes or phonological centralization of vowels in unstressed positions.