I can remember this from a long time ago, playing about with it on my palm IIIx. It was quite fun as I recall. Sadly the link to download this is broken, but I am sure I have it on a disc somewhere. I’ll have to dig it out to add it to my OS4 collection.
Anyway, here’s the blurb:
The Harmonic Square is a revolutionary new musical instrument conceived by theoretical physicist and inventor Nick Charlton. It has a set of 25 keys arranged in a square, each one playing a harmonic of a particular base note. This arrangement enables the player to produce a far richer selection of harmonics of a given note than an ordinary keyboard.
The Harmonic Square is more like a 2-dimensional keyboard. Keys are one of two colours. The dark keys correspond to notes that already exist on a keyboard. The light keys correspond to new harmonics that cannot be produced on an ordinary instrument.