How was The Original of Laura composed before Vladimir Nabokov’s death?

Vladimir Nabokov composed his final novel, The Original of Laura, on a stack of index cards rather than in a traditional manuscript format.1

At the time of his death in 1977, the work remained unfinished. Although Nabokov instructed his wife, Véra, to burn the cards, she chose to preserve them in a Swiss bank vault instead. The novel was eventually published in 2009 by the couple's son, Dmitri, who described the text as a concentrated distillation of his father's creative style.1