Frettle is a free, open source app that shows every note, chord shape and scale on the neck of a guitar, bass, ukulele or mandolin. Pick a chord to see all the ways to play it up the neck, with the root and intervals marked and barres drawn in. Pick a scale to see it across the whole fretboard, split into hand-sized groups, along with the chords that fit it. Tap any note to hear it.