The Bocca di Leone (Lion's Mouth) was a postbox with a difference. It was the means by which one Venetian could secretly denounce another, if he or she were suspected of breaking the law.
It is often thought that the denunciations were anonymous, but this was not the case. A note had to be signed and also include the signatures of two witnesses to his or her good character. Anonymous accusations were meant to be burnt, but the information so provided could be acted upon, if it involved matters concerning state security.
There was a financial incentive for citizens to snitch on one another, as those making a denunciation were rewarded for their efforts, if their information proved correct.
Bocche di Leone were once to be found throughout the city, but only a few have survived.