In phonetics, liquids or liquid consonants are a class of consonants consisting of lateral consonants together with rhotics.
Liquids as a class often behave in a similar way in the phonotactics of a language: for example, they often have the greatest freedom in occurring in consonant clusters. In some languages, such as Japanese, there is one liquid phoneme which may have both lateral and rhotic allophones.
English has one lateral, /l/ and one rhotic, /r/, and therefore has two liquids, exemplified in words such as led and red. Most other European languages also have two liquids, corresponding to /l/ and /r/ respectively.
Some European languages, such as Italian and Serbo-Croatian, have more than two liquid phonemes. These languages typically have the set /l/ /ʎ/ /r/, though some (like Russian) have /lʲ/, /ɫ/, /r/ (Russian also has /rʲ/).
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. In North America, a majority of languages do not have rhotics at all and there is a wide variety of lateral sounds – though most are obstruent laterals rather than liquids. Most indigenous Australian languages are very rich in liquids, with some having as many as seven distinct liquids. These typically include dental, alveolar, retroflex and palatal laterals, and as many as three rhotics. This richness in liquid consonants in a sense compensates for the small vowel inventories and lack of fricatives of Aboriginal languages.
On the other side, there are many indigenous languages in the Amazon Basin and eastern North America with no liquids, and also a few in Asia and Africa. Polynesian languages typically have only one liquid, which may be either a lateral or a rhotic.
The grammarian Dionysius Thrax used the Greek word ὑγρος (hugros, "moist") to describe the /l,r,m,n/ phonemes of classical Greek. Most commentators assume that this referred to their "slippery" 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, whence it has been retained in the Western European phonetic tradition.
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