In theoretical linguistics, underspecification is a phenomenon in which certain features are omitted in underlying representations. Restricted underspecification theory holds that features should only be underspecified if their values are predictable. For example, in most dialects of English, all front vowels (/i, ɪ, e, ɛ, æ/) are unrounded. It is not necessary for these phonemes to include the distinctive feature [−round], because all [−back] vowels are [−round] vowels, so the roundness feature is not distinctive for front vowels. Radical underspecification theory, on the other hand, also allows for traditionally binary features to be specified for only one value, where it is assumed that every segment not specified for that value has the other value. For example, instead of the features [+voice] and [−voice], only [+voice] is specified and voicelessness is taken as the default.
The concept of underspecification is also used in morphological theory, particularly to refer to cases in which a morpheme does not bear an entire set of feature-values, and is thus compatible with a wide range of potential morphological environments. In this approach to morphology, for example, while the English pronouns he vs. she are specified for gender, the plural pronoun they would be underspecified for gender.
Example of underspecification in phonology
In Tuvan, phonemic vowels are specified with the articulatory features of tongue height, backness, and lip rounding. The archiphoneme |U| is an underspecified high vowel where only the tongue height is specified.
Whether |U| is pronounced as front or back and whether rounded or unrounded depends on vowel harmony. If |U| occurs following a front unrounded vowel, it will be pronounced as the phoneme /i/; if following a back unrounded vowel, it will be as an /ɯ/; and if following a back rounded vowel, it will be an /u/. This can be seen in the following words: