[DavidPeiffer]

This reminds me of a neat piece of computer keyboard -> audio software I found on what had to be an "old internet" site 15-20 years ago. For lack of a better phrase, it was relative tone keyboard. I've looked but have not been able to find the software, not remembering any hint of the name, but it was fun to play with.

It worked one of two ways, I'm not positive which.

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You stared with musical note C. One note could be played at a time. G would go down a half note, H up a half note. F down a whole note, J up a whole note. Repeatedly pressing G would go down the chromatic scale. Playing a Diatonic scale up would be a combination of pressing H and J.

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Pretend the keyboard letter G is the base note, mapped to C in music. F would give a half note lower, H a half note higher, and so on across the home row of the keyboard. Then you could adjust the base note (perhaps T to go down a half note, Y to go up a half note).

In essence, you could transpose a song from the key of C to D by doing a modifier, and your fingers could complete the exact same sequence. In a jazz application, something on Spiral Synth like "FSA, GDS, HFD, K" might have been

[pierrec]

Jeskola Relativion does exactly what you describe. Relatively obscure because it's exclusively a Buzz plugin and it's not documented anywhere. I believe it comes with Buzz, it's mentioned in the changelog here: https://jeskola.net/buzz/beta/files/changelog.txt

That said, it's a fairly simple thing to develop, and I wouldn't be surprised if there are a bunch of other implementations.