One of the most intuitive distinctions which feature theory needs to capture is that between consonants and vowels. There are three features, the so-called major class features, which provide a rough first grouping of sounds into functional types that includes the consonant/vowel distinction.

syllabic (syl): forms a syllable peak (and thus can be stressed).

sonorant (son): sounds produced with a vocal tract configuration in which spontaneous voicing is possible.

consonantal (cons): sounds produced with a major obstruction in the oral cavity.

The feature [syllabic] is, unfortunately, simultaneously one of the most important features and one of the hardest to define physically. It corresponds intuitively to the notion “consonant” (where [h], [ j], [m], [s], [t] are “consonants”) versus “vowel” (such as [a], [i]): indeed the only difference between the vowels [i, u] and the corresponding glides [ j, w] is that [i, u] are [+syllabic] and [ j, w] are [-syllabic]. The feature [syllabic] goes beyond the intuitive vowel/consonant split. English has syllabic sonorants, such as [r̩ ], [l̩], [n̩ ]. The main distinction between the English words (American English pronunciation) ear [ɪr] and your [ jr̩ ] resides in which segments are [+syllabic] versus [-syllabic]. In ear, the vowel [ɪ] is [+syllabic] and [r] is [-syllabic], whereas in your, [ j] is [-syllabic] and [r̩ ] is [+syllabic]. The words eel [il] and the reduced form of you’ll [ jl̩] for many speakers of American English similarly differ in that [i] is the peak of the syllable (is [+syllabic]) in eel, but [l̩] is the syllable peak in you’ll.

Other languages have syllabic sonorants which phonemically contrast with nonsyllabic sonorants, such as Serbo-Croatian which contrasts syllabic [r̩ ] with nonsyllabic [r] (cf. groze ‘fear (gen)’ versus gr̩ oce ‘little throat’). Swahili distinguishes [mbuni] ‘ostrich’ and [m̩ buni] ‘coffee plant’ in the fact that [m̩ buni] is a three-syllable word and [m̩ ] is the peak (the only segment) of that first syllable, but [mbuni] is a two-syllable word, whose first syllable peak is [u]. Although such segments may be thought of as “consonants” in one intuitive sense of the concept, they have the feature value [+syllabic]. This is a reminder that there is a difference between popular concepts about language and technical terms. “Consonant” is not strictly speaking a technical concept of phonological theory, even though it is a term quite frequently used by phonologists – almost always with the meaning “nonpeak” in the syllable, i.e. a [-syllabic] segment.

The definition of [sonorant] could be changed so that glottal configuration is also included, then the laryngeals would be [–sonorant]. There is little compelling evidence to show whether this would be correct; later, we discuss how to go about finding such evidence for revising feature definitions.

The feature [sonorant] captures the distinction between segments such as vowels and liquids where the constriction in the vocal tract is small enough that no special effort is required to maintain voicing, as opposed to sounds such as stops and fricatives which have enough constriction that effort is needed to maintain voicing. In an oral stop, air cannot flow through the vocal tract at all, so oral stops are [–sonorant]. In a fricative, even though there is some airflow, there is so much constriction that pressure builds up, with the result that spontaneous voicing is not possible, thus fricatives are [–sonorant]. In a vowel or glide, the vocal tract is only minimally constricted so air can flow without impedance: vowels and glides are therefore [+sonorant]. A nasal consonant like [n] has a complete obstruction of airflow through the oral cavity, but nevertheless the nasal passages are open which allows free flow of air. Air pressure does not build up during the production of nasals, so nasals are [+sonorant]. In the liquid [l], there is a complete obstruction formed by the tip of the tongue with the alveolar ridge, but nevertheless air flows freely over the sides of the tongue so [l] is [+sonorant].

The question whether r is [+sonorant] or [-sonorant] has no simple answer, since many phonetically different segments are transcribed as r; some are [-sonorant] and some are [+sonorant], depending on their phonetic properties. The so-called fricative r of Czech (spelled ř) has a considerable constriction, so it is [-sonorant], but the English type [ɹ] is a sonorant since there is very little constriction. In other languages there may be more constriction, but it is so brief that it does not allow significant buildup of air pressure (this would be the case with “tapped” r’s). Even though spontaneous voicing is impossible for the laryngeal consonants [h, ʔ] because they are formed by positioning the vocal folds so that voicing is precluded, they are [+sonorant] since they have no constriction above the glottis, which is the essential property defining [+sonorant].

The feature [consonantal] is very similar to the feature [sonorant], but specifically addresses the question of whether there is any major constriction in the oral cavity. This feature groups together obstruents, liquids and nasals which are [+consonantal], versus vowels, glides, and laryngeals ([h, ʔ]) which are [-consonantal]. Vowels and glides have a minor obstruction in the vocal tract, compared to that formed by a fricative or a stop. Glottal stop is formed with an obstruction at the glottis, but none in the vocal tract, hence it is [-consonantal]. In nasals and liquids, there is an obstruction in the oral cavity, even though the overall constriction of the whole vocal tract is not high enough to prevent spontaneous voicing. Recent research indicates that this feature may not be necessary, since its function is usually covered as well or better by other features.

The most important phonological use of features is that they identify classes of segments in rules. All speech sounds can be analyzed in terms of their values for the set of distinctive features, and the set of segments that have a particular value for some feature (or set of feature values) is a natural class. Thus the segments [a i r̩ m̩ ] are members of the [+syllabic] class, and [ j h ʔ rmsp] are members of the [-syllabic] class; [a r̩ j ʔ r m] are in the [+sonorant] class and [s z p b] are in the [-sonorant] class; [aiwh ʔ] are in the [-consonantal] class and [r̩ m̩ r m s p] are in the [+consonantal] class. Natural classes can be defined in terms of conjunctions of features, such as [+consonantal, -syllabic], which refers to the set of segments which are simultaneously [+consonantal] and [-syllabic].

When referring to segments defined by a combination of features, the features are written in a single set of brackets – [+cons, -syl] refers to a single segment which is both +consonantal and -syllabic, while [+cons] [–syl] refers to a sequence of segments, the first being +consonantal and the second being -syllabic.

Accordingly, the three major class features combine to define five maximally differentiated classes, exemplified by the following segment groups.

Further classes are definable by omitting specifications of one or more of these features: for example, the class [-syllabic, +sonorant] includes {j, w, h, ʔ, r, l, m}.

One thing to note is that all [+syllabic] segments, i.e. all syllable peaks, are also [+sonorant]. It is unclear whether there are syllabic obstruents, i.e. [s̩ ], [k̩ ]. It has been claimed that such things exist in certain dialects of Berber, but their interpretation remains controversial, since the principles for detection of syllables are controversial. Another gap is the combination [-sonorant, -consonantal], which would be a physical impossibility. A [-sonorant] segment would require a major obstruction in the vocal tract, but the specification [-consonantal] entails that the obstruction could not be in the oral cavity. The only other possibility would be constriction of the nasal passages, and nostrils are not sufficiently constrictable.

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