Mozart's original music for organ consists of 17 one-movement "Epistle sonatas" - chamber music written between 1772 and 1780 for masses in Salzburg. The instrumentation for 14 of these is two violins and basso continuo, while the remaining three are written for more instruments. The pieces are short - this is music written to be played between the reading of texts. In 1790 Mozart was commissioned to write music for what was known as a "Flötenuhr" - a large grandfather clock containing a self-playing organ. The works are compositions of exceptionally high musical quality. Here the mature Mozart reveals his gifts as a contrapuntalist and his ability to create singable melodies and forward-looking harmonies.