I've read in two books that the correct posture of the hands when playing piano are that of holding an orange in the hand.

But many of the great virtuosos seem to use different hand postures when playing.

The shape the hands take varies as you play. The best default is slightly curved as they are when hanging at your side. The holding an orange stuff is 100 years old.

I would also like to point out we all have different hand shapes. I have really tiny hands and I'm sure that would make how I hold my hands look different from someone with really large hands.

Hi Gomtorus,

A very good question indeed.
What I have learned during the teaching process:

What is really important about the posture:
The chair should be at such a height that elbows are about the same level with keys or a little bit lower.

The sitting bones are the biggest connecting point of the body and chair.

Backbone is in a balanced position so that muscules do not have to compensate the wrong support of backbone.

Also the pianist should not be too far from or too near to the keyboard.

There are the main things.

I have discovered that if fingers are put to work in a right way and finger technique is good, there are no problems with hand position. It all comes naturally and looks really right.

Of course, to achieve the right finger technique is quite a bit of work

The connection point between the pianist and the piano is a very important moment. And it determines a lot. I think that what ever too fixed position is not good.

The idea and musicality can transform into music through the pianist and instrument the best way, if human body is flexible and free.

Best wishes,
Jaak

Search for Taubman videos on YouTube why players shouldn't curve their fingers that is/was so widely taught. There's another video about sitting heights and how that affects your playing. There's also a video of her answering the question about "am I too old for this". I think once you watch a video, you'll want to watch them all!