This short handbook wants to make some mysteries concerning the enormous amount of pictures (especially gold backgrounds, Holy Virgins enthroned with child, angels and Saints and cherubs, predellae of Altarpieces and altars, diptychs and polyptichs) Pietro Lorenzetti, the most famous painter of the Middle Ages in Siena together with Giotto, Duccio di Boninsegna e Simone Martini, had created all through his not too long life.In this text I herald the recondite affairs that were basic for the comprehension of this puzzle.Pietro, when he was very old and no more able to travel for painting through all Italy, made up his mind to adopt his best pupil and afford to him the chance to sign all the chefs d'oeuvres with his own signature, till his death, and baptized him Ugolino Lorenzetti.I deem the enlightenment is quite rocambolesque and many other examples of similar phenomena in Art History are enrolled in this short handbook.