It's called a piano stool. Because the piano player has requirements for posture and stability of the stool, the stool should be made into this shape. First, make it round, so that it can be done in a small range, and it is easy to sit unsteadily when playing the piano. Secondly, because the piano keyboard is longer, it is made into a rectangle. When playing the piano, you should try to sit up straight and try not to tilt your body too much, so make the stool into a rectangle to play around on the piano. Considering the height of the pianist, some piano benches can be raised and lowered. There are various designs of piano benches, all of which are for the players to play the piano conveniently and correctly.