I went through and read your design document - thanks for the link! That answered my question. For anyone else wondering, one piano key per character within a given modifier palette (like letter, number, etc). With 24 major and minor chords, 4 times that in minor and major sevenths, another 12 dominant chords, 12 augmented, and 12*11 interval pairs, there is a very large space to map interface structures to. The trick is making it harmonically pleasing - however, with a graph of harmonic transitions and cadences, it ought to be possible to find isomorphisms that preserve pleasing sound if you matched unikely letter combinations to unpleasing sounds. Perhaps in the same way that i9en used to infer what you had typed based on probability and scrolling, perhaps we could do something similar with arpeggios or chords played within a certain span.