Margen
For more than a century, Allegri's Miserere could only be heard in the Sistine Chapel during Holy Week. The Vatican considered it so sacred that copying the score was forbidden — punishable, it was said, by excommunication.
In April 1770, a fourteen-year-old Mozart attended one of those performances with his father. Back at their lodgings, he wrote out the entire work from memory: some twelve minutes of music for nine voices and two choirs. On the Friday he went back to hear it again, the manuscript hidden in his hat, to correct a few small details.
We know all this because his father, Leopold, described it in a letter to his wife, equal parts proud and worried about the consequences. There were none: months later, Pope Clement XIV decorated the boy. And Rome's best-kept secret was undone by a prodigious memory.