If you play a string instrument and want to learn the notes on each strings, the notes on music staff, etc. the FretTrainer APP is good.
There are 6 instruments available to practice on: Guitar, Bass, 5-String Bass, Mandolin, Ukulele, Banjo.
https://www.theguitarjournal.com/fret-trainer-app-now-on-i…/
![](https://i1.wp.com/www.theguitarjournal.com/wp-content/uploads/2018/11/Fret-Trainer.jpg?w=690&ssl=1)
It is free, but has ads.