La consonant fricativa alveolar sonora es transcriu [z] en l'AFI, és a dir, la lletra zeta minúscula. Està present a la majoria de llengües però no als idiomes escandinaus. En algunes llengües, com el castellà, no té valor de fonema i és només una variant de [s].
En català és l'anomenada essa sonora. Pot estar representada per la lletraZ ("zero"), per una S entre vocals ("casa") o per les grafies típiques de [s] quan es produeix una sonorització d'aquell fonema. La seva existència data de l'edat mitjana, ja que el llatí no contenia cap so equivalent a [z].[1][2]
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