Ponchielli set a libretto by Arrigo Boito, published under the anagram Tobia Gorrio, drawn from Victor Hugo's play Angelo, tyran de Padoue. The opera opened at La Scala on 8 April 1876, and Ponchielli revised it repeatedly over the following four years; the version of 1880 became standard. Each of its four acts carries a title, and its Venice is a place of masks, denunciations and staged entertainment — including the Dance of the Hours, which long ago outgrew the work around it. It remains the only opera by its composer in general circulation, and among the most demanding for its six principals.