At various points in Venice you'll see the 'Lion's Mouths" that were used by Most Serene Republic of Venice for collecting citizen's complaints anonymously. The one on the left is for the Sestiere of Dorsoduro. Venice is divided into the six sestieri (or districts.)
Inside the Doge's palace is a special Lion's Mouth that's like a direct line to the ancient equivalent of the IRS. We saw several others that were just simple slots in the walls with labels, but these two were the most picture-esque.
Denunciations against people harming the public health for the Sestiere of Dorsoduro
Report secrets against those who offer bribes and officials who collude in order to hide their true unearned income.
While the lions’ mouths were the people's pipeline to the authorities, they probably weren't a place where someone could rat you out anonymously and wait for the Council of Ten to haul you away without any investigation or evidence of wrongdoing, despite what Mark Twain has to say:
"These were the terrible Lions’ Mouths. These were the
throats down which went the anonymous accusation thrust in secretly in the dead
of night by an enemy, that doomed many an innocent man to walk the Bridge of
Sighs and descend into the dungeon which none entered and hoped to see the sun
again." - Mark Twain, Innocents Abroad
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