A castrato was a male soprano or contralto whose unbroken voice had been artificially preserved by surgical castration before puberty. By this means the vocal folds were prevented from the normal thickening which causes them to produce the lower notes in an adult male voice.
A side effect of the operation was to retain the agility of a boy's vocal folds, producing in the adult remarkable flexibility as well as strength and voluptuousness in tone.
Boy singers were originally castrated in response to an interpretation of a statement in the writings of St. Paul which declared that women should keep silent in church. As a result the castrato voice became available to composers of opera seria.
A fashion for castrati in Italian opera led to the creation of many r├┤les which survive in the repertoire as breeches parts. So popular were males in these r├┤les at the time of their composition that it's said that female singers were driven to posing as men in order to get work.
Some idea of the power and virtuosity of the castrato voice can be had from the works of composers such as Handel and his contemporaries. However, hearing r├┤les written for castrati sung by countertenors, by women, or transposed down for tenors cannot give a true indication of the specific castrato timbre.
As late as the early nineteenth century Rossini was composing for Velluti, the last great castrato, for whom Meyerbeer was still writing in 1824.
Wagner, when composing Parsifal, considered casting a castrato for the part of the self-emasculated character, Klingsor. However, he abandoned the idea before the opera's eventual production in 1882.
The last professional castrato was Alessandro Moreschi, who died in 1922. He did not sing in opera, but a number of his recordings remain to give us an idea of the unique qualities of the castrato voice.
