History beguiles us with tales of secret ornamentation - the so called abbellimenti - never written down, but simply passed from performer to performer in the Papal Chapel. It was de rigeur for those on the Grand Tour in the 18th century to hear the work in the Sistine Chapel during Holy Week. Many have expounded on the piece's beauty and uniqueness, and legend tells that unauthorised copying of the work was an excommunicable offence. Despite this, Mozart is supposed to have copied the work after hearing it performed, although knowing Mozart, it is neither surprising that he was able to copy the work, nor that he risked the consequences. No copy survives by his hand.
Although rumours of the work's inauthenticity abound, very few people have heard the work performed differently, and even fewer will be so bold as to suggest exactly what a replacement should be. That having been said, many people will prefer the 'top C' version, and not care whether it is authentic or not.
My intention is to look firstly at the manuscript sources for this work in both the Biblioteca Apostolica Vaticana and the British Library, see how the piece has been written throughout its history, and then draw conclusions from the music alone. A few imponderables remain, upon which I have hypothesised. Finally, an authentic edition is produced, to show in modern notation how the work would have been sung.