Guitars are even worse, due to fretting. The act of fretting a note stretches the string, which raises its pitch, and the amount varies by string and by fret. So guitars are never truly in tune. Not only are harmonics not sufficient, but neither are electronic tuners! A guitar that is "in tune" according to an electronic tuner is always sharp in practice.
And it gets worse from there! The tuning of a guitar string varies as it decays. This is true of piano strings too, but not to the same degree. A freshly struck string is vibrating more widely than a gentle or decaying note, so it's stretching itself sharp. You can see this on a fast digital tuner - the note will go sharp at first, and then settle to a slightly lower pitch. So when you tune to an electronic tuner, are you tuning the initial pitch, or the decayed pitch? This variation might be 20 cents or more.
Because of this, how to tune a guitar well is very much a matter of taste, context, and experience. I use harmonics and electronic tuners to get myself in the ballpark, then start fine-tuning based on the guitar itself (each one has its own quirks), and the material I'm planning to play. On acoustic guitar, I tend to focus on getting the B string in tune with the D and A strings first, by the quality of octaves for open C and D chords (which also gets the A and D in tune with each other). Then I focus on getting the low E in tune with an octave E on the D string. Then get the high E in tune in unison with E on the B string. Finally, get the G string in tune with G on the low E, an octave down. This means my G string is usually a bit flat relative to the D and B strings, but that's okay - it's in tune for G chords, and being a little flat is good for E major and D chords. I might adjust a little if I'm playing in C/Am.
James Taylor has an excellent YouTube video about tuning guitars consistently with electronic tuners. It's very much to his taste and the specific guitars he uses, but his principles are sound. And if you try it on an acoustic with good intonation, you'll immediately hear that "James Taylor" sound.
You're welcome! The most important takeaway, I think, is that to a certain degree, "in tune" is a matter of opinion (for fretted instruments, anyway). You can significantly change the tonal quality of the instrument while remaining "in tune". That's the neat thing about the James Taylor video... follow his method, and your guitar suddenly gets that James Taylor sound, very rich and resonant.
Try experimenting with modal tunings like DADGAD, too. It's much easier to get them "in tune", and you hear this beautiful resonance that guitars can make.
I brought this up on Facebook, and a friend who is an excellent player responded with his own tuning method. He tunes the A string to a reference (tuner, piano), and then tunes every other string to a fretted A note that is in tune with the open A string. This is probably more "in tune" than the highly resonant approach that I use.
And it gets worse from there! The tuning of a guitar string varies as it decays. This is true of piano strings too, but not to the same degree. A freshly struck string is vibrating more widely than a gentle or decaying note, so it's stretching itself sharp. You can see this on a fast digital tuner - the note will go sharp at first, and then settle to a slightly lower pitch. So when you tune to an electronic tuner, are you tuning the initial pitch, or the decayed pitch? This variation might be 20 cents or more.
Because of this, how to tune a guitar well is very much a matter of taste, context, and experience. I use harmonics and electronic tuners to get myself in the ballpark, then start fine-tuning based on the guitar itself (each one has its own quirks), and the material I'm planning to play. On acoustic guitar, I tend to focus on getting the B string in tune with the D and A strings first, by the quality of octaves for open C and D chords (which also gets the A and D in tune with each other). Then I focus on getting the low E in tune with an octave E on the D string. Then get the high E in tune in unison with E on the B string. Finally, get the G string in tune with G on the low E, an octave down. This means my G string is usually a bit flat relative to the D and B strings, but that's okay - it's in tune for G chords, and being a little flat is good for E major and D chords. I might adjust a little if I'm playing in C/Am.
James Taylor has an excellent YouTube video about tuning guitars consistently with electronic tuners. It's very much to his taste and the specific guitars he uses, but his principles are sound. And if you try it on an acoustic with good intonation, you'll immediately hear that "James Taylor" sound.