When you re-string your guitar, it isn’t enough to just tune it up. You also need to fine-tune the natural harmonics to help ensure your guitar “sounds right” up and down the fretboard. Most guitarists test their harmonics at the 12th fret. Others also test the open-string naturals at the 9th and 7th and 5th frets and sometimes the 19th fret. It can take a careful touch to get the harmonic to sound — let alone ring right — but the process of divining those harmonics so your guitar can vibrate in tune is of great, melodic, importance in creating memorable music.