It's not often that my years of messing about with text encoding comes in useful in my life as a harmonica player, but once in a while it happens. The harmonica is one of the many instruments for which traditional music notation doesn't always give you enough information about what to play. Any note on the staff can be played on one and only one key of a piano, for instance, but can be played in any of a dozen or so places on a guitar. So guitarists often use tablature that places numbers on lines representing each string, telling them which string to play and exactly where to fret it.