What is a Zero Fret?
A zero fret is simply another fret positioned where a standard nut would be situated. It allows much better control of intonation and string action. The zero fret takes care of the intonation and, more importantly, establishes the correct and minimal string height. There’s no better height than being identical to those other frets. So, our string passes over a zero fret before it goes through the captured nut.
Because of the zero fret, it keeps the Voyage-Air Guitar tone more consistent. Open strings sound just like a fretted note. They're usually found on expensive guitars because there is more work involved with slotting for the zero fret and setting it up correctly.