[erhe]

Now that I think about it, the test results could even be parsed into a 9261-entry lookup table and it would cover all possible transpositions of all possible notes, without having to even care about what the logic actually is. Unless you also want to transpose from/to double-flats, double-sharps etc.

[erhe]

So we can just change the logic to look at the key of the chord and if it is a sharp key, make the bass note sharp as well (and the same for flats)?

There is no "key of the chord". Chord symbols are not in a key. You do not have to have a key to play a chord, and an A major chord will be exactly the same regardless of what keys anyone is thinking or writing about. Atonal, keyless music can use chord symbols. What do you do if the song is keyless? Having a key or not does not matter.

And whether there are flats or sharps in a key signature, does not affect transposition of notes in any way. Because it does not matter if you even have a key signature to begin with. Note names in chord symbols are absolute, they do not depend on key or key signature, or the lack thereof. You do not even need to care about the fact that they are chords. You take every note name one by one, whether it is on the left or right side of a slash, and transpose each of them by the exact same interval and the exact same logic throughout the piece.

[sciurius]

Good work, @erhe, thanks for providing these excellent reference materials.

I think that in case of MobileSheets and ChordPro, the much simpler yet effective approach that Mike mentioned in #14 will suffice.

Note that there is a difference between key transpose, and interval (semitones) transpose. ChordPro uses semitones transposition. This means that it is not possible to transpose from e.g. F# to Gb since the interval is zero. You can, however transpose from F# to G and then from G to Gb. But that is different matter.

[sciurius]

There is no "key of the chord"

It is just a way of saying that the bass notes should be named according to the signature corresponding to the chord.

[sciurius]

You take every note name one by one, whether it is on the left or right side of a slash, and transpose each of them by the exact same interval and the exact same logic throughout the piece.

That's even easier ☺. So you're happy with Bb/D → A/Db ?

[erhe]

To get A from Bb, you transposed down by a minor second, interval (-1, -1) for my transposition routine. Applying the same transposition to D yields C#. An interval of a second means that the letter changes by one position in the list of letters.

To get Db from D would be something like a "diminished unison", (0, -1).

I don't understand what you mean by "signature corresponding to the chord". When transposing slash chords, you perform the same process to both sides of the slash. The bass note is its own independent entity, when in comes to transposing. The transposing of what's on the right side is in no way affected by what is on the left side. If the bass note made sense in the untransposed version, but not in the transposed one, then you messed up the transposing somehow.

[sciurius]

Let's see if I can follow.

To get A from Bb, you transposed down by a minor second,

Yes, a minor second = one interval.

Applying the same transposition to D yields C#.

One interval lower than D is the note between C and D, which is Db or C#, depending on the scale.

Since we do not take key signatures into account, I'm inclined to call this Db. What musical rule says it should be called C# ?

[BRX]

I'm just talking from the end users perspective, but as a pianist often working form the chords I just want to say A/Db feels wrong to me in any circumstance.

I don't think you can or should disregard the "key signature" or rather scale of the chord. If it becomes an A by way of transposing or otherwise the root/slash chords should be from the scale of the new chord (so always a C# for the A chord, never a Db).

[Skip]

I think normal convention is to use sharps for all notes/chords based (rooted) on natural (A>G) and sharp scales (A#>G#) and flats on notes/chords based on flat scales (Ab>Gb). That suggests the key of the 'too' scale controls the construction of the transposed chord. In the case of a 'keyless' 'from', the construction of the chord suggests the key type. With a keyless 'too', I suppose that would depend on the user with the restriction of 'one or the other' for the complete tune.

A mechanical approach that may work is to make two circles of the chromatic scale spacing each note, separating the sharps and flats and natural notes equidistant (17 notes). Make one circle smaller to fit inside the larger one. The smaller one will rotate inside the larger. The outer is the 'from', the inner is the 'too'. This will result in a straight one to one relationship.
G to A transposition---G>A, G#>A#, Ab>Bb, A>B, A#>C, Bb>C#, etc.