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In linguistics, a distinctive feature is the most basic unit of phonological structure that may be analyzed in phonological theory.
Distinctive features are grouped into categories according to the natural classes of segments they describe: major class features, laryngeal features, manner features, and place features. These feature categories in turn are further specified on the basis of the phonetic properties of the segments in question.
Since the inception of the phonological analysis of distinctive features in the 1950s, features traditionally have been specified by binary values to signify whether a segment is described by the feature; a positive value, [+], denotes the presence of a feature, while a negative value, [−], indicates its absence. In addition, a phoneme may be unmarked with respect to a feature. It is also possible for certain phonemes to have different features across languages. For example, [l] could be classified as a continuant or not in a given language depending on how it patterns with other consonants.
In recent developments[when?] to the theory of distinctive features, phonologists have proposed the existence of single-valued features. These features, called univalent or privative features, can only describe the classes of segments that are said to possess those features, and not the classes that are without them.
[+/− sonorant] This feature describes the type of oral constriction that can occur in the vocal tract. [+son] designates the vowels and sonorantconsonants (namely glides, liquids, and nasals), that are produced without an imbalance of air pressure in the vocal tract that might cause turbulence. [−son] describes the obstruents, articulated with a noticeable turbulence caused by an imbalance of air pressure in the vocal tract.
Laryngeal features: The features that specify the glottal states of sounds.
[+/− voice] This feature indicates whether vibration of the vocal folds occurs with the articulation of the segment.
[+/− spread glottis] Used to indicate the aspiration of a segment, this feature denotes the openness of the glottis. For [+sg], the vocal folds are spread apart widely enough for frication to occur; for [−sg], there is not the same friction-inducing spreading.
[+/− constricted glottis] The constricted glottis features denotes the degree of closure of the glottis. [+cg] implies that the vocal folds are held closely together, enough so that air cannot pass through momentarily, while [−cg] implies the opposite.
[+/− continuant] This feature describes the passage of air through the vocal tract. [+cont] segments are produced without any significant obstruction in the tract, allowing air to pass through in a continuous stream. [−cont] segments, on the other hand, have such an obstruction, and so occlude the air flow at some point of articulation.
[+/− nasal] This feature describes the position of the velum. [+nas] segments are produced by lowering the velum so that air can pass through the nasal tract. [−nas] segments conversely are produced with a raised velum, blocking the passage of air to the nasal tract and shunting it to the oral tract.
[+/− strident] The strident feature applies to obstruents only and refers to a type of friction that is noisier than usual. This is caused by high energy white noise.
[+/− lateral] This feature designates the shape and positioning of the tongue with respect to the oral tract. [+lat] segments are produced as the center of the tongue rises to contact the roof of the mouth, thereby blocking air from flowing centrally through the oral tract and instead forcing more lateral flow along the lowered side(s) of the tongue.
[+/− delayed release] This feature distinguishes stops from affricates. Affricates are designated [+del rel]
[+/− anterior]: Anterior segments are articulated with the tip or blade of the tongue at or in front of the alveolar ridge. Dental consonants are [+ant], postalveolar and retroflex ones are [−ant].
[+/− distributed]: For [+dist] segments the tongue is extended for some distance in the mouth. In other words, laminal dental and postalveolar consonants are marked as [+dist], while apical alveolar and retroflex consonants are [−dist].
[+/− high]: [+high] segments raise the dorsum close to the palate. [−high] segments do not.
[+/− low]: [+low] segments bunch the dorsum to a position low in the mouth.
[+/− back]: [+back] segments are produced with the tongue dorsum bunched and retracted slightly to the back of the mouth. [−back] segments are bunched and extended slightly forward.
[+/− tense]: This feature (mainly) applies to the position of the root of the tongue when articulating vowels. [+tense] vowels have an advanced tongue root. In fact, this feature is often referred to as advanced tongue root (ATR), although there is a debate on whether tense and ATR are the same or different features.
The concept of a distinctive feature matrix to distinguish similar elements is identified with phonology, but there have been at least two efforts to use a distinctive feature matrix in related fields. Close to phonology, and clearly acknowledging its debt to phonology, distinctive features have been used to describe and differentiate handshapes in fingerspelling in American Sign Language. Distinctive features have also been used to distinguish proverbs from other types of language such as slogans, clichés, and aphorisms.
Analogous feature systems are also used throughout Natural Language Processing (NLP). For example, part-of-speech tagging divides words into categories. These include "major" categories such as Noun vs. Verb, but also other dimensions such as person and number, plurality, tense, and others. Some mnemonics for part-of-speech tags conjoin multiple features, such as "NN" for singular noun, vs. "NNS" for plural noun, vs. "NNS$" for plural possessive noun (see Brown Corpus). Others provide more explicit separation of features, even formalizing them via markup such as the Text Encoding Initiative's feature structures. Modern statistical NLP uses vectors of very many features, although many of those features are not formally "distinctive" in the sense described here.
^Godsave, Bruce. 1974. An investigation of the feasibility of using a particular distinctive feature matrix for recording and categorizing fingerspelling errors. University of Cincinnati, doctoral dissertation.
^p. 73. Norrick, Neal. 1985. How Proverbs Mean: Semantic Studies in English Proverbs. de Gruyter.