[1][2] 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 (see below).
[1][3] This word was calqued into Latin as liquidus (possibly because of a mistranslation),[3] whence it has been retained in the Western European phonetic tradition.
[6] Thus Czech and other Slavic languages allow their liquid consonants /l/ and /r/ to be the center of their syllables – as witnessed by the classic tonguetwister strč prst skrz krk "push (your) finger through (your) throat."
[8] In Latin and Ancient Greek, obstruent + liquid consonant clusters (known as muta cum liquida)[5] supposedly were ambiguous in this sense, and as such were often used to manipulate meter.
[11] 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,[12][13] and they are more likely to be deleted in consonant clusters before the age of three.
"[16] To better determine the full range of articulatory and acoustic characteristics of liquids, the use of ultrasound paired with audio recordings is increasing.
On an auditory level, liquid consonants resemble each other, which is likely the reason they undergo or trigger assimilation, dissimilation and metathesis.
This phenomenon, which is not so common worldwide, is attested in Finnish: e.g., the root tul- "to come" combined with the past participle suffix -nut, yields the surface form tullut.
[18] For example, the genitive of the Ancient Greek noun ἀνήρ anḗr "man" is ἀνδρός andrós, with the insertion of a [d] sound between a nasal consonant and the liquid [r].
Another example is the Irish word bolg "belly", usually pronounced with an epenthetic schwa [ə] after the liquid [lˠ]: [ˈbˠɔlˠəg].
Liquids can often be the result of lenition,[18] the change of a consonant towards characteristics that are typical of vowels, making it "weaker".
Elsewhere in the world, two liquids of the types mentioned above remains the most common attribute of a language's consonant inventory except in North America and Australia.
On the other side, there are many indigenous languages in the Amazon Basin and eastern North America, as well as a few in Asia and Africa, with no liquids.