Since the number of speech sounds in the world's languages is much greater than the number of letters in any one alphabet, linguists have devised systems such as the International Phonetic Alphabet (IPA) to assign a unique and unambiguous symbol to each attested consonant.
The word consonant comes from Latin oblique stem cōnsonant-, from cōnsonāns 'sounding-together', a calque of Greek σύμφωνον sýmphōnon (plural sýmphōna, σύμφωνα).
[1][2] Dionysius Thrax calls consonants sýmphōna (σύμφωνα 'sounded with') because in Greek they can only be pronounced with a vowel.
[a] He divides them into two subcategories: hēmíphōna (ἡμίφωνα 'half-sounded'),[4] which are the continuants,[b] and áphōna (ἄφωνος 'unsounded'),[5] which correspond to plosives.
One blurry area is in segments variously called semivowels, semiconsonants, or glides.
On one side, there are vowel-like segments that are not in themselves syllabic, but form diphthongs as part of the syllable nucleus, as the i in English boil [ˈbɔɪ̯l].
On the other, there are approximants that behave like consonants in forming onsets, but are articulated very much like vowels, as the y in English yes [ˈjɛs].
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] foot.
Some pairs of consonants like p::b, t::d are sometimes called fortis and lenis, but this is a phonological rather than phonetic distinction.
The approximant /w/ is also widespread, and virtually all languages have one or more nasals, though a very few, such as the Central dialect of Rotokas, lack even these.
A few languages on Bougainville Island and around Puget Sound, such as Makah, lack both of the nasals [m] and [n] altogether, except in special speech registers such as baby-talk.
[e] Despite the 80-odd consonants of Ubykh, it lacks the plain velar /k/ in native words, as do the related Adyghe and Kabardian languages.
[f] For instance, an areal feature of the Pacific Northwest coast is that historical *k has become palatalized in many languages, so that Saanich for example has /tʃ/ and /kʷ/ but no plain /k/;[11][12] similarly, historical *k in the Northwest Caucasian languages became palatalized to /kʲ/ in extinct Ubykh and to /tʃ/ in most Circassian dialects.