Songfacts®:

• In 1638 Gregorio Allegri, an Italian singer, priest and composer, wrote "Miserere," which is a simple, part-harmonised setting of Psalm 51. This piece, which is also called "Miserere mei, Deus" (Latin: "Have mercy on me, O God") was composed by Allegri for use in the Sistine Chapel during matins, as part of the exclusive Tenebrae service on Wednesday and Friday of Holy Week. So potent were its vocal stratospherics that the Vatican kept the score under lock and key. Writing it down or performing it elsewhere was punishable by excommunication.
• Gregorio Allegri (1582 - 1652) lived mainly in Rome, where he would later die. He was one of the earliest composers for stringed instruments, but the Miserere is by far his most celebrated composition. Allegri was known for his dedication to charity work, daily visiting prisoners and others in need.
• The 14-year-old Mozart who was touring Italy as a child wonder, arrived in Rome in 1770 and was invited to a liturgical celebration in the Sistine Chapel, where he could listen to the Miserere for two nine-part choirs. Aware that he could not get the music score because it was strictly prohibited, Mozart transcribed the piece in its entirety from memory, only returning a second time to correct minor errors. It is now established that Mozart almost certainly knew the work beforehand. Some wags have referred to it as the first ever bootleg.