Among the musical compositions of Allegri were two volumes of concerti, published in 1618 and 1619; two volumes of motets, published in 1620 and 1621; besides a number of works still in manuscript. He was one of the earliest composers for stringed instruments, and Athanasius Kircher has given one specimen of this class of his works in the Musurgia. But the most celebrated composition of Allegri is the Miserere, still annually performed in the Sistine Chapel at Rome. It is written for two choirs, the one of five and the other of four voices, and has obtained a celebrity which, if not entirely factitious, is certainly not due to its intrinsic merits alone.
The mystery in which the composition was long enshrouded, no single copy being allowed to reach the public, the place and circumstances of the performance, and the added embellishments of the singers, account to a great degree for much of the impressive effect of which all who have heard the music speak. This view is confirmed by the fact that, when the music was performed at Venice by permission of the pope, it produced so little effect that the emperor Leopold I, Holy Roman Emperor., at whose request the manuscript had been sent, thought that something else had been substituted. In spite of the precautions of the popes, the Miserere has long been public property.
In 1769 Mozart heard it and wrote it down, and in 1771 a copy was procured and published in England by Dr Burney. The entire music performed at Rome in Holy Week, Allegri's Miserere included, has been issued at Leipzig by Breitkopf and Härtel. Interesting accounts of the impression produced by the performance at Rome may be found in the first volume of Felix Mendelssohn's letters and in Miss Taylor's Letters from Italy.
Initial text from a 1911 encyclopedia, update as needed