The Sibilant reference article from the English Wikipedia on 24-Apr-2004
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Sibilant

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In phonetics, a sibilant is a consonant produced by "hissing" (from the Latin verb sibilare, "to whistle"). It is a kind of linguistic catchall, but useful because sibilant sounds tend to migrate between sibilant classes when languages evolve, and not really to other classes: the Proto-Germanic verb meaning to swim has split into the English swim, with the sibilant s, and the German schwimmen, with the English sibilant sh.

Examples of sibilants in English include the sounds represented by s in stick and sh\ in shark. English contrasts two types of sibilants; some dialects of Inuktitut use just one; the Caucasian language Ubykh makes contrast between four types of sibilant; and Rotokas, an Austronesian language, lacks sibilants altogether.