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Coaching me in the Venetian pronunciation, he explained that the word itself was a local invention. In La Serenissima’s glittering heyday, correspondents signed letters, “Il Suo schiavo” (“your slave”). Meeting on the street, acquaintances would bow and repeat the same ingratiating words. However, in the Venetian dialect, which softens the hard sound of sch (pronounced sk in other regions) to a chewy sh (as in “show”), Suo schiavo came out sciao, which melted into ciao as it migrated to other parts of Italy.
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Dianne Hales (La Bella Lingua: My Love Affair with Italian, the World's Most Enchanting Language)