Any speech sound characterized by an articulation in which a closure or narrowing of the vocal tract completely or partially blocks the flow of air; also, any letter or symbol representing such a sound. Consonants are usually classified according to the place of articulation (e.g., palate, teeth, lips); the manner of articulation, as in stops (complete closure of the oral passage, released with a burst of air), fricatives (forcing of breath through a constricted passage), and trills (vibration of the tip of the tongue or the uvula); and the presence or absence of voicing, nasalization, aspiration, and other features.
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Since the number of consonants in the world's languages is much greater than the number of consonant letters in any one alphabet, linguists have devised systems such as the IPA (IPA) to assign a unique symbol to each attested consonant. In fact, the Latin alphabet, which is used to write English, has fewer consonant letters than English has consonant sounds, so digraphs like "ch", "sh", "th", and "zh" are used to extend the alphabet, and some letters and digraphs represent more than one consonant. For example, many speakers are not aware that the sound spelled "th" in "this" is a different consonant than the "th" sound in "thing". (In the IPA they are transcribed ð and θ, respectively.)
However, even Plato's original conception of consonant is inadequate for the universal description of human language, since in some languages, such as the Salishan languages, stop consonants may also occur without vowels (see Nuxálk), and the modern conception of consonant does not require cooccurrence with vowels.
All English consonants can be classified by a combination of these features, such as "voiceless alveolar stop consonant" [t]. In this case the airstream mechanism is omitted.
Some pairs of consonants like p::b, t::d are sometimes called fortis and lenis, but this is a phonological rather than phonetic distinction.
One blurry area is in segments variously called semivowels, semiconsonants, or glides. On the one side, there are vowel-like segments which are not in themselves syllabic, but which form diphthongs as part of the syllable nucleus, as the i in English boil [ˈbɔɪ̯l]. On the other, there are approximants which behave like consonants in forming onsets, but are articulated very much like vowels, as the y in English yes [ˈjɛs]. Some phonologists model these as both being the vowel /i/, so that the English word bit would phonemically be /bit/, beet would be /bii̯t/, and yield would be phonemically /i̯ii̯ld/. Similarly, foot would be /fut/, food would be /fuu̯d/, wood would be /u̯ud/, and wooed would be /u̯uu̯d/. However, there is a (perhaps allophonic) difference in articulation between these segments, with the [j] in [ˈjɛs] yes and [ˈjiʲld] yield and the [w] of [ˈwuʷd] wooed having more constriction and a more definite place of articulation than the [ɪ] in [ˈbɔɪ̯l] boil or [ˈbɪt] bit or the [ʊ] of [ˈfʊt].
The other problematic area is that of syllabic consonants, that is, segments which are articulated as consonants but which occupy the nucleus of a syllable. This may be the case for words such as church in rhotic dialects of English, although phoneticians differ in whether they consider this to be a syllabic consonant, /ˈtʃɹ̩tʃ/, or a rhotic vowel, /ˈtʃɝtʃ/: Some distinguish an approximant /ɹ/ that corresponds to a vowel /ɝ/, for rural as /ˈɹɝl/ or [ˈɹʷɝːl̩]; others see these as the a single phoneme, /ˈɹɹ̩l/.
Other languages utilize fricative and often trilled segments as syllabic nuclei, as in Czech and several languages in Congo and China, including Mandarin Chinese. In Mandarin, they are historically allophones of /i/, and spelled that way in Pinyin. Ladefoged and Maddieson call these "fricative vowels" and say that "they can usually be thought of as syllabic fricatives that are allophones of vowels." That is, phonetically they are consonants, but phonemically they behave as vowels.
Many Slavic languages allow the trill [r̩] and the lateral [l̩] as syllabic nuclei (see Words without vowels), and in languages like Nuxalk, it is difficult to know what the nucleus of a syllable is (it may be that not all syllables have nuclei), though if the concept of 'syllable' applies, there are syllabic consonants in words like /sx̩s/ 'seal fat'.