Bach wrote 15 each two-part inventions and three-part inventions, the latter of which are usually called "sinfonias" now. They were probably teaching pieces for the keyboard, possibly for his own children. You will notice on this manuscript copy that it says "Präambulum" which means "prelude" and raises a whole other question: prelude to what? The right hand of the keyboard part is also in soprano clef, which was usual at the time. This transcription for soprano and tenor recorders is by an unknown arranger and is very literal with only some octave adjustments as needed to fit it onto the instruments, which shows just how simple but terribly well-crafted it is.