In the spring of 1770, a father and his teenage son arrived in Rome during Holy Week. The father was Leopold Mozart, a musician from Salzburg. The son was Wolfgang Amadeus Mozart, fourteen years old and already famous across Europe as a child prodigy.

They went to the Sistine Chapel to hear a piece of music that few people outside the Vatican had ever heard written down. That night, Wolfgang did something that became one of the most famous stories in music history.

The guarded psalm

The music was a Miserere — a setting of Psalm 51, which begins "Have mercy on me, O God". It was written around the 1630s by Gregorio Allegri, a priest and composer who sang in the papal choir.

The Miserere was sung during the Tenebrae services of Holy Week, in the days before Easter. Candles were put out one by one until the chapel was in darkness, and the choir sang in the shadows. The piece alternated between simple chanted verses and sections of rich harmony for two choirs, one of five voices and one of four.

What made it special was not just the written notes. The papal singers decorated the music with elaborate ornaments — runs, flourishes and soaring high notes — that were passed down by tradition and not written in the ordinary score. The effect was said to be overwhelming.

The Vatican treated the piece as its own. Copies were rarely allowed out. Over time, a legend grew that anyone who copied it or performed it elsewhere could be punished with excommunication.

The boy who listened

According to the family's letters, Wolfgang heard the Miserere on the Wednesday of Holy Week. Afterward, he went back to their lodgings and wrote it out from memory. A few days later, he returned to hear it again, taking his manuscript with him to correct a few details.

Leopold described the episode in a letter home to his wife, saying they had the piece and asking her not to tell anyone, in case it caused trouble. "We have it," he wrote. Word spread anyway.

No punishment, but a medal

Mozart was not excommunicated. Instead, later that year, Pope Clement XIV made him a knight of the Order of the Golden Spur, an honour few musicians received. Some versions of the story say the pope was impressed by his musical genius, including the Miserere episode. There is no clear evidence that the two were connected.

The Miserere itself soon escaped the Vatican. In 1771, the English music historian Charles Burney published a version of it in London. Burney had visited Italy and obtained copies, perhaps from several sources. Although historians debate exactly where his version came from, the secret was out.

The high C

Today, Allegri's Miserere is one of the most popular pieces of choral music in the world. Many recordings feature a treble or soprano voice soaring up to a high C, a stunning moment that listeners wait for.

But that high C may not have been part of the music Mozart heard. Musicologists have traced it to a mistake made in the 19th century. When different sources were combined, a section was transposed into a higher key, apparently by error, and the result was a dramatic leap to a top C. The mistake was copied and became part of the tradition.

Was it really secret?

Historians point out that the Miserere was not completely hidden. A few copies had been given to important people over the years, including royalty. And the legend of excommunication may be exaggerated, or simply untrue. What was truly guarded was the performance tradition of the Sistine choir, with its unwritten ornaments.

That makes Mozart's feat even more remarkable. He wasn't just copying notes. He was capturing a living performance, by ear, after hearing it once.

A story worth retelling

The story of Mozart and the Miserere is one of the best-known tales of musical genius. It shows how music was once passed down, by listening and memory as much as by paper. And it is a reminder that many of the pieces we love have changed over time, shaped by errors, traditions and the people who handed them on.