Theremin

[MLE]

Also, it's very hard to get a linear note progression. Like:

A------B--C-------D-----E
The above would be like a cheap theremin


A----B----C-----D----E
Now that one would be much higher quality.


By the way, every theremin feels different, so once you learn on a specific one, it's very difficult to switch. It's like the keys on the piano change shape or move with each one you try to play.