Thumb pick
General mountain dulcimer or music discussions
When i used to play chord/melody style (about 100 years ago it seems, lol) I built up a really hard and durable callus on the side of my thumb right next to the nail. It protected everything, even the edge of my nail. Once in a while the outer layer would shed off naturally, but the layers under it quickly rebuilt to take its place.
It's funny how my left hand calluses evolve and change over time depending on what instrument I'm currently playing a lot, and in what style. Since i now play dulcimer type instruments with a noter, I don't get calluses from dulcimer playing. But I get banjo-playing calluses on the tips of my fingers, especially my left middle finger and my left index.