 During this time, the horn used was essentially a natural horn, which restricted the range of notes that were able to be easily used to only those in the instrument's natural harmonic series, and variations thereof created by using the hand in the bell to alter the pitch.German musicians also used an "Inventionshorn", which allowed some further range of notes by manually inserting extra crooks.Stölzel dedicated himself to the further development of the instrument, and experimented with adding valves that redirected the air stream into different lengths of tubing, to lengthen the sections of tubing available and thereby created more (and lower) usable harmonic series.