[jlg23]

I agree that a teacher is invaluable, but for training your muscle memory (and sheet reading, if one does not already), apps can be a good-enough substitute: Instant feedback for almost free, visual aids that even a teacher could not provide, 24/7 availability.

Of course, some physical basics (like posture for piano, breathing for wind instruments) are ideally established correctly right from the start and that's where even a few hours spent on professional help will pay off.

Things I paid a teacher for (flute) that could have been easily checked by an app:

* tone: the right one? does it come out clean?

* pace: am I keeping the pace correctly? (a metronome does not flash red when I am out of sync, the 11 year old me could ignore it really well..)

* loudness

And last but not least: Apps work everywhere, lessons with teachers are hard to get in some places. When my parents decided we move, I had the choice between a 2h commute for a 45 minute lesson or to wait about 1.5 years for a place with a local teacher (despite having given solo concerts already). I stopped playing and regret it to this day.