An interesting (at least to me) blog post on Why are D-sharp and E-flat considered to be two different notes
This confusion applies to all of the black keys, but in this post, I’ll be talking about the one between D and E. You could think of it as a raised D, in which case it’s called D-sharp. You could also think of it as a lowered E, in which case it’s called E-flat. Guitars don’t have black and white keys, so when I was a feral self-taught musician, I just thought of that note as the eleventh fret on the E string, the sixth fret on the A string, the first fret on the D string, etc. I pretty much always called it E-flat, regardless of context. I have since learned to use the correct name depending on context, but it still feels arbitrary sometimes, especially outside of diatonicism. If you are in B major, the note is supposed to be called D-sharp, and if you are in B-flat major, the note is supposed to be called E-flat. But what if you’re in A blues? How are you supposed to spell it then? And what difference does it make anyway?And then goes into a rather long but somehow understandable (to a person who knows math well, but not music, so much) explanation as to why they really were different notes. But as he points out, it really doesn't matter so much to fretted instruments, where you can't really have two different notes. So I sent this my personal musical assistant, Alex, and asked how this affected the guitar playing and he replied:
The usual answer is that you are only supposed to use each letter name once in any given scale. All major scales are considered to be based on C major, and you are supposed to preserve the white keys’ names, modified by accidentals as needed. So for example, you spell the B major scale like so: B, C-sharp, D-sharp, E, F-sharp, G-sharp, A-sharp. If you were to spell it as B, D-flat, E-flat, E, G-flat, A-flat, B-flat, it would sound exactly the same, but it would be harder to read, and you would lose points on your music theory exam.
Maybe spelling B major using flats instead of sharps is hard to read in notation, but what musical difference does it make? In the present day, the answer is, none whatsoever. Five hundred years ago, however, it would have made a very big difference. Before the advent of temperament systems, D-sharp and E-flat were two different notes. They weren’t just written differently; they sounded different. You can compare the historical versions of these notes yourself in this track I made. . . .
Well theoretically you can tune a guitar kind of mean tempered, and there are guitars with microtonal frets to play the difference between say c# and Db. But in actual practice, good players (I'd like to think I could) bend notes slightly all the time to make things more in tune on equal tempered guitar tunings. But sometimes if I'm recording something, i will mess with the tuning to get the 3rds more in tune, it works if you're playing something that stays in key and you don't do much other than open chords