I'm not understanding the accordion notation like this, e.g. G7/E7 or A/C# where each pair is written below a single note D and C# respectively. (If these had been written for guitar then I would have called them slash chords)
I took a screen shot of the wrong copy of the same piece. The notation that gave me a headache was this one below.
See bar three which I now understand was an ABC error by whoever transcribed the music. It should not read G7/E7 but should be E7/G# and so the counterbase can be played. That was not the only error, as the Scottish Tome was compiled by Tom Buchanan not Joe Buchanan!
Here is the original which I have just found as written by the composer:
They're called slash chords on accordion too. And, as you may know from guitar, they're not always the counterbass. They're used whenever the bass note to use with the chord is not the root. For example F/G (play regular G bass with the F major chord button).
I use slash chords with the piano as well, mostly on quick and sloppy "notes to self" when improvising or arranging and when I don’t need or am too lazy to take the time to write out sheet music (and don’t want to memorize everything!)
For example this is a bit of my scratch notes for something I play only once a year, “I’ll Be Home For Christmas”. I write the words, sometimes reminders of some of the melody notes, and most of the chords. The slashes are mostly for low LH bass notes. Maybe some day I’ll write this one out again so it will actually be legible!
I work this one up just before Christmas then forget it till the next year. I make notes like this for some favorite hymns too.
When asked to do several pieces for a church audience years ago with (an amazing) friend on trumpet, I used the same method but written larger and much neater, the words prominent with the chord reminders written above, a bit like fake books but without the music staff. While we were both more-or-less improvising, we would actually [gasp!] practice several times, almost like real musicians.
Same rule, except the added notation now even shows you at what note they want you to hit the designated chord. Depending on the composer it could also mean the accompaniment that is supposed to happen by a 2nd musician
Look at just this measure: View attachment 10153
The "B" is played with the E7th, the "C" played with the F#m7 and "D" played with the C7th chord and G# bass. This is a more complex set of what I like to call compound chords as it may take 3-4 fingers of the left hand to accomplish some of these chords.