Each verse began as plain chant - a single unison vocal line - then opened out like a flower, with the voices intertwining in glorious polyphony.
Custom forbade any other choir to sing it and anyone who broke this ban was liable to be excommunicated. Sheet-music was also not allowed to be circulated.
In 1770, a 14-year-old boy in a scarlet suit (trimmed with gold braid and lined with sky-blue satin) was brought to the Sistine Chapel by his father and afterwards he went home and wrote down the gist of this complex work with its nine-part harmonies. A few days later, with his copy hidden in his hat, he went back intending to make corrections, but the choir sang a different setting. On Good Friday they repeated it, however, and he emerged with a perfect copy.
His mother was worried: might he incur the wrath of the Pope? His father was enormously proud. "As it is one of the secrets of Rome, we do not wish to let it fall into other hands," he wrote home. Thus did Leopold Mozart and his son Wolfgang bring their Italian tour to a triumphant climax.
The Pope was impressed rather than affronted and invited Wolfgang to perform for the Sistine choir.
The work has had an afterlife. The version that we hear in cathedrals today and on "AllegriLassusPalestrina" (EMI 7243 5 75560 26), with its celebrated high "C" for the treble soloist, owes its high pitch to the fact that 20th-century editors followed the version Felix Mendelssohn copied down when he heard the work 60 years after Mozart. The old copies in the Vatican archives were lower and successive generations of choristers had turned it into a vocal stunt.