Back to school time.
Is it a fret or should we consider it a nut?
It's a fret. But it does the job of a nut. Sort of. Just to round off the sections on nuts and frets, let's have a chat about…
In some guitars and basses, a zero fret is installed. The zero fret sits where the nut would have been and the nut itself gets shifted, slightly, towards the headstock. The zero fret takes over the job of being the 'bearing point'. More or less by default, it handles the job of setting string height while the nut's job is relegated to just controlling side-to-side spacing.
Just to (try) clear up confusion I may have caused, moving the nut, in this case, isn't the same as what might happen with a Buzz Feiten or another compensated nut. The zero fret sits at the same place a regular nut would have been. Since the strings sit on that zero fret, compensation is unaffected. On a zero-fret guitar, the nut gets moved towards the headstock and it's job is now to align the strings before they pass over that zero fret. Clear as mud? Please feel free to shout up in the comments if I haven't explained this well.
Why would I want a Zero Fret?
Well, there is a pretty good argument around tonal consistency. Consider that most of the notes you play on your guitar have the string bearing against a metal fret as it sounds. On an instrument with a conventional nut, the open note doesn't bear against a fret but against one of those materials we've talked about before. Even with a great nut, that certainly has the potential to produce a slightly different tone.
From a luthier's point of view, a zero fret makes for a perfect setup as it pretty much takes care of itself for string height. No mucking about, trying to file nut-slots until they're the right height for optimal playability and intonation—just level all the frets and there you go. The nut part behind the zero fret doesn't need the same finicky height adjustment.
It's worth calling attention to a weird perception that zero frets are a mark of a cheap instrument. That's not the case at all. A number of cheap (and awful) instruments from the distant past had zero frets but that's not what made those instruments awful.
If your guitar or bass has a zero fret, you can disregard the worry about nut-materials and tone and just get on with things.
Bottom Line: Is It Worth Upgrading?
Hmmm. It's not really something that people worry about. While it's technically possible (with some caveats) to move from a regular nut to a zero fret or vice-versa, I'm not aware of anyone feeling strongly enough to want to do so. I wouldn't loose sleep about it.
If you've found this useful, you can check out others in the same series of Guitar Hardware School. Feel free to share these on and shout up in the comments if you've questions.