Constanze (far left in the above daguerreotype) was a trained musician and played a role in her husband's career. Two instances can be given:
The extraordinary writing for the soprano solo in the Great Mass in C minor (for example, in the "Christe eleison" section of the Kyrie movement, or the aria "Et incarnatus est") was intended for Constanze, who sang in the 1783 premiere of this work in Salzburg. Maynard Solomon in his Mozart biography speculatively describes the work as a love offering.
During the period of the couple's courtship, Mozart began making visits to Baron Gottfried van Swieten, who let him examine his extensive collection of manuscripts of work by Bach and Handel. Mozart was excited by this material, and prepared a number of compositions in Baroque style himself. An important impetus was Constanze, who apparently fell in love at this time with Baroque counterpoint. This is known from a letter Mozart wrote to his sister Nannerl on 20 April 1782. The letter was accompanied by a manuscript copy of the composer's Fantasy and Fugue, K. 394.
"I composed the fugue first and wrote it down while I was thinking out the prelude. I only hope that you will be able to read it, for it is written so very small; and I hope further that you will like it. Another time I shall send you something better for the clavier. My dear Constanze is really the cause of this fugue's coming into the world. Baron van Swieten, to whom I go every Sunday, gave me all the works of Händel and Sebastian Bach to take home with me (after I had played them to him). When Constanze heard the fugues, she absolutely fell in love with them. Now she will listen to nothing but fugues, and particularly (in this kind of composition) the works of Händel and Bach. Well, as she has often heard me play fugues out of my head, she asked me if I had ever written any down, and when I said I had not, she scolded me roundly for not recording some of my compositions in this most artistically beautiful of all musical forms and never ceased to entreat me until I wrote down a fugue for her."
-W.A. Mozart