Mozart returned to the simpler, chamber-sized scoring of two violins, bass, and organ in this, the final of the 17 Church sonatas. In No. 14 (K. 278) and No. 16 (K. 329), he had expanded the instrumentation into the orchestral realm, breaking from the ensemble made up of the four instruments listed above. No. 12 features that instrumentation, too, but with the addition of two trumpets. While No. 17 is scaled down in its forces, it is expanded in scope and length, lasting about five minutes, arguably making it the longest of any of the Church sonatas. It is also unusual in that the organ dominates it as in no other work in the series. It even has an extended cadenza-like solo near the end. This work opens with violins introducing the lively, graceful theme, after which the organ takes it up, imparting no sense of the sacred realm in the chipper vivacity of its upward scales and colorful swirls. There follows considerable development of the theme and a substantial reprise of the material. The work closes brilliantly after the aforementioned organ solo, with the orchestra having the last say.

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