Shakespeare’s primary source for Measure for Measure was The Right Excellent and Famous Historye of Promos and Cassandra: Divided into Commercial Discourses, by George Whetstone, published in 1578. Whetstone had derived his plot from the Italian author Cinthio’s collection of stories called The Hecatommithi, written in 1565. Although we have no direct proof that Shakespeare could read Italian, based on the frequency with which he used Italian works as source material, some of which were not even translated into English at the time, it is likely that Shakespeare did know Italian. In this case he probably read both Whetstone’s version and the original work of Cinthio.
Shakespeare follows the events in Whetstone’s text closely, including the comic and bawdy sub-plot involving Mistress Overdone and her bordello. But Shakespeare seems to have concluded that certain elements of Whetstone’s play were unsuitable for his dramatic purposes. In Promos and Cassandra, Cassandra does marry Promos, her Svengali and the murderer of her brother. This union would have been looked upon very harshly by Renaissance play-goers. As Davis Harding points out, “It was clearly his recognition of this flaw which led Shakespeare to make his most striking alteration in the plot of the original story. By introducing the Mariana story and the device of the bed-trick, he contrives to save the virtue of his heroine Isabella and, at the same time, does no disservice to Mariana, whose love for Angelo, it is made clear, had triumphantly survived the passage of time and his original dislike for her” (Harding, 129).