Robert Browning used the poetic device of the dramatic monologue in his poem "My Last Duchess." One advantage of using this device is that it allows the speaker's own words to reveal, celebrate, or, in this case, condemn his behavior. The speaker in "My Last Duchess" is the Duke, loosely based on the historical figure, the Duke of Ferrara whose own young wife died under mysterious circumstances. Browning writes the poem completely from his perspective and voice, allowing the reader to draw his or her own conclusions about the Duke's monologue.
In the opening stanza, the Duke introduces the painting of his last duchess. Using iambic pentameter thoughout his entire monologue, the Duke presents himself as a man who likes to be in control. His duchess, however, flouted that control by smiling too easily at other members of the household. Browning's use of dramatic monologue reveals the inner thoughts of the Duke as he discusses his last, unsatisfactory marriage; his wife was too liberal with her affections, so he had her killed:
"Oh sir, she smiled, no doubt,
Whene'er I passed her; but who passed without
Much the same smile? This grew; I gave commands;
Then all smiles stopped together."
Through the convention of dramatic monologue, Browning creates a character that reveals himself to be jealous, controlling, and sadistic; rather than describing these characteristics, hearing the speaker's own words gives the reader a first hand encounter with the psychopathic Duke.