Commentary Purg XVIII 145

Dante's purgatorial dreams are described in cantos IX, XIX, and XXVII, but the second occurs here, in a single line, 'e 'l pensamento in sogno trasmutai' (and I transformed my musings into dream).  Since in Vita nuova he presents his age as having been nine, eighteen, and twenty-seven for his three main 'encounters' with Beatrice, the poet perhaps wanted to retain those three nine-based and nine-spaced numbers for his three dreams that lead back to her now.  For this calculation see Hollander (Holl.1969.1), p. 145.