I'm planning on writing a piece that uses a lot of harmonics and I was wondering, how do you play harmonics in specific keys? How can I know what harmonics fit in a certain key, and do most harmonics (without a capo) fall under a certain key?
You need to know what notes are in what key. Then figure out what note each harmonic is.
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If you know how harmonics work, you can play tapped harmonics, which aren't that hard to learn, and make them fit in any key.

For instance, if you're playing in F#m and you want an F# harmonic, fret the 2nd fret of the high E string and tap the fretwire between the 14th and 15th frets on that same string, and BAM, you have an F# harmonic.
Really all you have to do is just make sure you know what notes fall into which keys.

If you're going to be playing harmonics using the open strings, I'm guessing you're going to be mainly using harmonics on the 12th fret, which you could use in the key of C-major, G-major and D-major (with their relative minors and what not), but like say if you go to the key of A-major, you couldn't play the harmonic G because the key of A-major contains G#s. So yeah, just know what the notes are of the harmonics, you could also use all of the harmonics on the 5th fret in the key of C-major and so fourth. Just figure out where you want to use the harmonics and we'll tell you if you can use them in the certain keys.
All of the harmonics on the 12, 7, 5, 19, and 24 frets are found in the G major scale and it's relative minor (E minor).
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