Wolfgang Amadeus Mozart is one of the world's best known child prodigies who grew up to become one of the great immortals of classical music. Mozart had an older sister called Nannerl. She too was a child prodigy, a marvellous singer and a remarkable harpsichordist, and like her brother was introduced by their father Leopold to all the European courts. At the end of three years of travel with the family, in Versailles she met the son of Louis XV who encouraged her to write music.

But Nannerl was a girl and in the 18th century a girl did not have the right to compose...