Lots of musicians have done this: Nick Drake, Joni Mitchel, Sonic Youth, the Velvet Underground and Blues musicians. Arguably, it’s modern usage was pioneered by the Delta Blues musicians, some of their tunings were more unusual than the more what we commonly think of as the blues tunings you come across on the internet. My point is, the blues pioneers did a lot of experimental stuff, especially early on. Alternate and unusual tunings are also common in non-western music and Bach was an innovator of their application in classical music.
In classical music it’s called “Scordatura” whose literal translation from Italian means “discord” or “mistuning.” It’s more commonly known as “cross-tuning” and in classical was often used on violins. Bach in particular would often use cross-tuning. Vivaldi also used it, you can here it in “The 4 Seasons.” In Vivaldi’s piece it provides that full, rich and deep tone. Cross tuning utilizes octaves, thus creating a natural chorusing effect from a single instrument.
In modern and noisier music, octaves create consonance which helps to balance out other dissonant harmonic combinations in the composition. This combination helps to infuse the music with tension, drama and a full body of sound that is multi-layered and can be manipulated in fun ways. Pairing unison notes together can create really wild sounds especially if exploited with an tube amp, few trusty pedals and/or a slide.
In some ways it simplifies things, you can do bar chords with one finger and with simple chord shapes create complex harmonic combination containing 7ths and upper extensions. I was doing this for years entirely ignorant of the theory behind it, you can definitely hear how the instrument is intoned with a different resonance. You have the ability to modulate your instruments voice by experimenting with tuning.
Does anybody else experiment with guitar tunings? I’ve come up with more than 20 of them, but didn’t write them all down. Through time I found 2 that I really like. From low to high: F#-A-D-A-A-D and D-A-D-A-A-D. You can tweek things easily too, if you lower the high D to a C# you have a Dmaj7 chord which creates an edgy dissonance and haunting harmonic tension that is also smoothed out by the octaves and unions notes. You can play with the dynamic between the two depending on when you add the major 7th (here, C#) That tuning is reminiscent of late 80s to early 90s era Sonic Youth. Often you can derive several tunings from one with simple adjustments and tweeks.
The tuning with the F# is an inverted D major triad, but also functions as a F#m with a flat 6, the sound is nice and moody. In this tuning, I never fret the bass string instead letting it ring, drone or hum open. I play off of the F# from the 5 strings below. However the D-A-D-A-A-D tuning is more compatible if you are playing with people that aren’t used to playing along with guitars tuned like that because it fits with Drop D and Open D perfectly. It doesn’t work well with guitars in standard unless they understand that your tonality will be misaligned with theirs and are able to adjust to that. As a note, I think it’s good to be ready to play with anybody in standard as a courtsey. Doing music with other people is a collective experience with a collective product so you have to able to converse in the same language. Or here, the same tuning.
I figured out these tuning by ear and only taught myself music theory some 20 years later. I was insecure about it for years, but it just sounded so good and people liked it. Later, I was delighted to find that the tunings made perfect sense according to theory and similar tunings had been used prodigiously by Bach, Vivaldi and others. This helped me gain confidence, because I just heard it and I knew then that I could trust my ears even if guitar bros would stare at me strange sometimes. I felt like the ugly duckling a lot of the time, lol.
When you play don’t be cerebral, just let it come out! You can analyze it later and build from there if you run into problems or are song writing. It only gets cerebral when you write or talk about it because speech and thought are a stand in for sound and space.
Theory is only a kind of grammar for music. It gives you tools and material to use while in the act of playing or composing. Music comes from the heart, from memory and the subconscious. Music theory stems from a select group of white, European men from centuries ago so don’t be afraid to break the rules and get weird with it! Blues and Jazz musicians created an entire new tonality by breaking the rules. That tonality is now a corner stone of modern music theory and the root of experimental music across many genres. Main thing though, trust your ears!