This is an excellent app, the only thing I would change is to maybe add a scale/arpeggio trainer, and to change the fact that in some modes you can only study four frets at a time. This is fine for guitar and bass, but mandolins use diatonic fingering and we play all the notes up to the seventh fret in first position. Fix the positioning for mandolin, and add more things to study, and this app would be perfect!