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The panoramic view on this double-page spread depicts Frascati from the terrace of the Villa Aldobrandini (also known as the Villa Belvedere). Roughly outlined on this page is the Cathedral, the Basilica of St Peter the Apostle, with its two bell-towers flanking the Baroque façade, whilst the opposite page contains part of the villa in the foreground, see folio 30 verso (D15352). The composition of the sketch is similar to that of a drawing by James Hakewill, View from the Terrace of the Belvedere Villa at Frascati 1817 (British School at Rome Library) which Turner would certainly have known.1
Tony Cubberley and Luke Herrmann, Twilight of the Grand Tour: A Catalogue of the drawings by James Hakewill in the British School at Rome Library, Rome 1992, no.5.26, p.251 reproduced.