Question: How should you place your hand while you're playing the piano?Oliver
Albert's reply: The hand should be curved in a smooth arc, roughly forming a half-circle.
If you place your hand flat on a table and slide your wrist forward while keeping your fingertips in place, the hand will naturally form the proper position.
Important: Make sure you don't collapse your knuckles! The major knuckles (where the fingers attach to the palm) must be the outermost point of the arc, and never pressed inward.
The anatomy of the hand is utilized in piano playing. We have three long fingers and two short ones. The long fingers, in the middle of the hand, most naturally play the shorter keys (the black keys), while the shorter fingers on the outside of the hand, the thumb and little finger, most naturally play the white keys.
Chopin recognized this fact and started each new student off by placing the longer fingers on the group of three black keys, with the thumb and little finger on the white keys E and B. (I use C in place of B for symmetry and because I have very large hands.) This is the natural piano hand position.