Which notes sound good together depends on the sound playing them. Every real sound is a stack of overtones, and when the overtones of two notes rub against each other, we hear roughness.
Here harmony is calculated from the sound itself: we measure each instrument's overtones and compute, for every pitch, how smoothly it blends. Nothing is looked up in a music-theory table, so a bell, a piano and a sine wave each get their own map of what fits.
Visual Harmonics is free & open source. Play, break things, and look inside.
Credits: dissonance model after Plomp & Levelt and W. A. Sethares. Orchestral samples: VS Chamber Orchestra: Community Edition by Versilian Studios / Sam Gossner & Ivy Audio / Simon Dalzell (CC0), vis.versilstudios.net/vsco-community.html. Instruments marked “local” are on this computer only and are not published.