Before playing piano for hours, you have to learn the correct piano posture, otherwise it could affect your health with cramps or fatigue, for example. But also you have to care about your hand position to improve your dexterity and performance.
The basic rule for playing piano is to sit up straight and never slouch. Sit on the front half of the bench, in front of the middle of the piano with your feet on the floor. Keep your back straight and your arms hang relaxed from your shoulder. Your elbows should be slightly in front of you (if the elbows are to your side or behind you, the bench is too close to the piano). Your elbows can be slightly higher than keys to better control the volume of your sound. It is important to adjust the bench height so that you are in the correct playing position at all times.
When you sit at the piano, first comes the relaxtion and then all the rest. Relax your shoulders, elbows, wrists and hands. When you place your hands on the keys, you must keep your hands arched and your fingers slightly curled at all times. Keep your wrists as flexible and relaxed as possible, this is the secret for achieving a deep and expressive sound.
You have learned the correct piano posture and how important it is to be relaxed in general and for your wrists in particular. At this point, you are ready to look a little closer at the piano.