Traditionally thought to have been the poetic composition that, apart from Queen Mab [See Separate Entry], took Shelley the longest time and caused him the greatest artistic difficulties, Prometheus Unbound was first conceived of in only three acts. It was not until April 1819 that Shelley decided to significantly expand and revise his lyrical drama into the completed four act version published in August 1820. Shelley begun writing the first act of his lyrical drama at Este in September 1818 and, then, worked sporadically on Acts II and III (substantially altered in March and April 1819) in the late autumn and winter 1818, and completed Act IV over the summer of 1819. These new dates for the composition of …
Sandy, Mark. "Prometheus Unbound". The Literary Encyclopedia. First published 25 August 2004; last revised 30 November -1.
[http://www.litencyc.com/php/sworks.php?rec=true&UID=2545, accessed 19 April 2015.]