I just had a little brainwave. I want to mark up a song text with diatonic autoharp chords. It's pretty simple, just Bm, A and D. Nobody but me needs to read it. Scrawling chord letters into a printed text looks messy.

I have key/colour synaesthesia. It sometimes comes in handy.

Highlighter pens. Bm - pale blue, A - green, D - yellow. No symbols, just streak through the text.

The same coding would work for anyone, but for me it's a natural code.