Could anyone out there please tell me the difference between music written in (say) the key of C or D. Is it just that in D all the notes are a tone higher? If so why bother. And why difficult keys like E flat? It can't be (as I have seen written) that certain keys are easier on some instruments; for example E is easier on the guitar than F; just retune the instrument! Perhaps on stringed instruments the open strings sound different to fretted strings and each chord thus sounds different rather than higher or lower. Am I right in saying that going up the scale the gap (in frequency) actually increases slightly between succesive notes - they're not laid out like sleepers on a rail way line. This would certainly explain it as a higher key wouldn't just be higher but the gaps between the notes would also be slightly different. Please solve a puzzel my music degree course daughter can't explain in a language that I can understand. Dr John (physicist!)