In some cases if the reed is defective you will tune and it will not stay in tune. I recently had my Vibrandoneon tuned and I had to replace a reed that would not stay tuned. If a reed has been stressed (blown to hard) it will tune then as it warms up fall out of tune. When tuning for chords like in a guitar you have overtones or harmonica that you tune to for a more full sound or sometimes a tighter sound depending on the tuning. If you have a guitar go close to the sound hole and barely touch the string then pluck you will here a ping or high pitch tone(harmonic) you can tune to that if you know the note and harmonic to another string below or above. This does not work for the melodica only string instruments that can sustain a tone. Tuning an instrument by ear requires a bit of practice and more training of the ear than the practice of scraping a reed. The first instrument I learned to tune was my Wurlitzer electric piano, not my favorite thing to do but back then it was a $35 dollar service charge just to open it and look, this was 1970 and $35 bucks was a lot of money so I learned to do it. Now I pay a $100 buck and they tune and adjust the entire Vibrandoneon, well worth it. To know how to tune your melodica is a good thing, I have a couple of beater melodicas that I am planning on practicing on so if I bust a reed que sera sers.