‘Fu qualche cosa d’incredibile …': ‘it was something unbelievable, something new, unprecedented’. Thus wrote Marianna Barbieri-Nini, the singer who created the role of Verdi's Lady Macbeth, in reference to the famous Act I duet between Macbeth and Lady Macbeth. Indeed, this duet seems to have been one of the most effective pieces in the opera for nineteenth-century audiences. Verdi himself wrote about it as one of the ‘two principal numbers’ of the opera, together with the sleepwalking scene, upon which the opera's dramaturgy depended. The reception of this piece at the 1847 première was stupendous. Antonio Calvi cites it as one of the opera's three excellent pieces, in which ‘maestro Verdi has arrived at Shakespeare’s sublimity'. The duet's opening section (‘Fatal mia donna’), according to various press releases during the opera's first few runs, was regularly encored three, four, even five times an evening. Abramo Basevi waxed eloquent over the piece, calling it ‘the culminating number of the opera’, ‘a true flash of Verdian genius’. And by the time the revised version was premièred in Paris in 1865, this duet had become entrenched in the repertory in a way the entire opera had not: one reviewer noted that the Act I duet ‘has always been considered the best duet Verdi has ever written’.