The Inventions and Sinfonias is a collection of short pieces in 2 and 3 voices Bach wrote for his students. These are among the finest examples of artistic gems ever written for this purpose, and probably because of this, they became very popular among Bach's students and others ever since they were written. The Invention No. 13 in A minor is a bold piece with a strong theme based on a broken A minor chord and dramatic development. It is, above all, a great exercise in broken chords and arpeggios.