In linguistics, a liquid consonant or simply liquid is a consonant class that consists of rhotics and laterals, which are also respectively described as "R-like" and "L-like" sounds. Liquids have also been defined as "non-nasal sonorant consonants" (although this definition often includes semivowels as well). The word liquid seems to be a calque of the Ancient Greek word ὑγρός (hygrós 'moist'), initially used by grammarian Dionysius Thrax to describe Greek sonorants. Liquid consonants are notable for several distinctive properties. They are more prone to be part of consonant clusters and of the syllable nucleus. Acoustically, their third formants are generally non-predictable based on the first two formants. Another important feature is their complex articulation, which makes them a difficult consonant class to study with precision and the last consonants to be produced by children during their phonological development. They are also more likely to undergo certain types of phonological changes such as assimilation, dissimilation and metathesis. Most languages have at least one liquid in their phonemic inventory. English has two, /l/ and /r/.
History and etymology The grammarian Dionysius Thrax used the Ancient Greek word ὑγρός (hygrós, 'moist') to describe the sonorant consonants ([l, r, m, n]) of classical Greek. It is assumed that the term referred to their changing or inconsistent (or "fluid") effect on meter in classical Greek verse when they occur as the second member of a consonant cluster. This word was calqued into Latin as liquidus (possibly because of a mistranslation) and this calque has been retained in the Western European phonetic tradition.
Sonority and syllable structure In the sonority hierarchy, liquids are considered the most sonorous sounds after vowels and glides, with laterals considered to be less sonorous than rhotics. This explains why they are more likely to be part of consonant clusters than other consonants (excluding glides), and to follow obstruents in initial consonant clusters and precede them in final consonant clusters. Liquids also hold this position in the hierarchy of syllable peaks, which means that liquids are theoretically more likely to be syllabic (or, in other words, be part of a syllable nucleus) than any other consonants, although some studies show that syllabic nasals are overall more favoured. Thus Czech, Slovak and other Slavic languages allow their liquid consonants /l/ and /r/ to be the center of their syllables – as witnessed by the classic tonguetwister in Czech strč prst skrz krk "push (your) finger through (your) throat." Additionally, Slovak also has long versions of these syllabic consonants, written ŕ and ĺ, e.g.: kĺb [kɫ̩ːp] 'joint', vŕba [ˈvr̩ːba] 'willow', škvŕn [ʃkvr̩ːn] '(of) spots'. This is also true for General American English (see the words barrel and anchor) and other English accents. Sequences of an obstruent and a liquid consonant are often ambiguous as far as syllabification is concerned. In these cases, whether the two consonants are part of the same syllable or not heavily depends on the individual language, and closely related languages can behave differently (such as Icelandic and Faroese). In Latin and Ancient Greek, obstruent + liquid consonant clusters (known as muta cum liquida) supposedly were ambiguous in this sense, and as such were often used to manipulate meter.
Acoustic and articulatory phonetics Acoustically, liquids seem to have a third formant of unexpected value when compared to the first and second formants. This contrasts with non-liquid approximants, whose third formant value is expected based on the first two formants. In articulatory phonetics, liquids are described as controlled gestures, which are slower and require more precise tongue movement during the "homing phase," when the tongue adjusts towards the place of articulation of the consonant. Due to the fact that babies prefer ballistic gestures, which rely on the propelling motion of the jaw, liquids usually occur later in a child's phonological development, and they are more likely to be deleted in consonant clusters before the age of three. Liquids have also been described as consonants involving "complex lingual geometries." Due to liquids being difficult to analyse on a purely auditory basis, the use of ultrasound paired with audio recordings is increasing in order to better determine the full range of articulatory and acoustic characteristics of liquids.
Sound changes Liquids seem to be more or less subjected to certain sound changes than other consonants. On an auditory level, liquid consonants resemble each other, which is likely the reason they undergo or trigger assimilation, dissimilation and metathesis.
Metathesis Cross-linguistically, liquids tend to be more prone to metathesis than other consonants, especially long-distance metathesis. In Spanish, a frequent example is the behaviour of /r/ and /l/:
Lat. crocodīlus > Span. cocodrilo 'crocodile' Lat. mīrāculum > Span. milagro 'miracle' Lat. perīculum > Span. peligro 'danger' Lat. parabola > Span. palabra 'speech' In English, comfortable is frequently pronounced /ˈkʌmf.tɚ.bəl/ in rhotic varieties, even though its stem, comfort, is pronounced /ˈkʌm.fɚt/, with the rhotic /ɹ/ in its original position.
Assimilation Liquid consonants can also undergo assimilation: compare Italian parlare "to speak" with Sicilian parrari. This phenomenon, which is not so common worldwide with respect to liquids, is attested in Finnish. For example, the root tul- "to come" combined with the past participle suffix -nut, yields the surface form tullut. This is one of the reasons long liquids are common in Finnish. A specific form of liquid assimilation, liquid harmony, is present is some languages. In Sundanese, some morphemes have two different realisations depending on what liquid is present in the root.
Dissimilation Liquids are also prone to dissimilation when they occur in sequence. For example, Old Italian colonnello 'colonel' is borrowed into Middle French as coronnel, which is in turn loaned into English as colonel, with an orthography inspired by Italian but with the /ˈkɚnəl/ or /ˈkɜːnel/ pronunciation with the rhotic r, which is absent in writing.
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