Instruments: violin, viola, cello, & piano
Level: beginner-early intermediate
Used for: finger shape
Buy here: https://amzn.to/2oW7M0I
Note: This is a tool just for fingers, not thumb, since none of the instruments I teach need a thumb oriented the way it would have to be in order to pop the bubble.
I start all of the above instrumentalists on just the index finger until the curve is consistent.
For piano players I then expand out to middle and ring fingers. Typically a beginner's pinky is too weak to maintain a good curve by itself, so I tend to stick with the middle three (piano fingers 2, 3, & 4) for beginning pianists. Later we practice landing fingers in pairs and then the pinky gets to play.
For violin, viola, and cello I add fingers cumulatively to help establish the concept of keeping fingers down. So one the pointer is staying nicely curved, we do string fingers 1 & 2 together, then 1, 2, & 3 simultaneously, and then all four.