Bach composed Two Part Inventions in K then (before 1723) for teaching purposes, particularly his eldest son Wilhelm Friedemann. The earliest copy of this work, supposedly written by Bach's own hand, is found in the Wilhelm Friedemann Clavierb chlein (Piano Notebook), started on January 22 1720. The Notebook contained two-part pieces, Preambulum, which were later named as Inventions. In Bach's copy of 1723 he prefaced the collection of Inventions...