Discuss the need for Distinctive Features
Distinctive feature theory was first formalized by Roman Jakobson in (1941) and remains one of the most significant contributions to phonology.

The use of distinctive features enables one to describe the segments in the world’s languages and to refer to those groups of segments that play a role in their characteristic phonological processes and constraints (Gussenhoven & Jacobs).

It is established that any language has a limited number of phonological contrasts or oppositions and given that no two languages are phonologically identical, distinctive features must be, to some extent at least, language specific.

A segment is specified

Via the use of distinctive features, phonemes are broken down into smaller components. For example, a nasal phoneme /m/ might be represented as a feature matrix [+ sonorant, -continuant, +voice, +nasal, +labial]. By representing /m/ in this way, we are saying both something about its phonetic characteristics- it is a sonorant, and it is a non-continuant because the airflow is totally interrupted in the oral cavity. It is important, however, to choose distinctive features that establish natural classes of phonemes. For example, since all the other nasal consonants and nasal vowels (if a language has them) have feature matrices that are defined as [+nasal], we can refer to all these segments in a phonological rule at the same time by making the rule apply to [+nasal] segments. Similarly, if we want our rules to refer to all the approximants and high vowels, we might define this natural class by [+sonorant, +high].

The advantage of this approach is readily apparent in writing phonological rules. For example, we might want a rule, which makes approximants voiceless when they follow aspirated stops in English. If we could not define phonemes in terms of distinctive features, we would have to have separate rules, such as [l] becomes voiceless after /k/ (\'claim\'), /r/ becomes voiceless after /k/ (\'cry\'), /w/ becomes voiceless after /k/ (\'quite\'), /j/ becomes voiceless after /k/ (\'cute\'), and then the same again for all the approximants that can follow /p/ and /t/. If we define phonemes as bundles of features, we can state the rule more succinctly as e.g. [+sonorant, -syllabic, +continuant] sounds (i.e. all approximants) become [+spread glottis] (aspirated) after sounds which are [+spread glottis] (aspirated).

If the features are well chosen, it should be possible to refer to natural classes of phonemes with a small number of features. For example, [p t k] form a natural class of voiceless stops in most languages: we can often refer to these and no others with just two features, [-continuant, -voiced]. On the other hand, [m] and [d] are a much less natural class (ie. few sound changes and few, if any, phonological rules, apply to them both and appropriately it is impossible in most feature systems to refer to these sounds and no others in a single feature matrix).