As far as I can tell I am seated properly. I have followed all the pictures and everything. I am trying to play Ode To Joy. I have my hand curved like I am holding a bubble. I have a large hand and am a large guy so bare in mind I am reaching around my girth. The problem I am having is that my fingers are almost hitting the black keys but my thumb is on middle C barely holding on to the tip of the key for dear life and it slips off a lot. Can someone diagnose what is wrong? Arms seem comfortable with forearms parallel to the floor and the wrist is straight except of course it is facing slightly left to put the thumb on middle C. I think I am sitting far enough away as only about an inch of my knees are under the keys and if I reach straight out to where the keys ed and raise my arms and break my wrists the are at 90 degrees to the start of the keys. I should mention that placing my hand directly in a straight line in front of me doesn't present this problem, but this would be at least two octaves up and down from middle C.

Hard for me to tell by the description and because I know so little,
Can you rest your hand in a 5 finger position with your thumb happily on middle C and your fingers each on neighboring white keys?

People's hands are different. I have a narrow and long palm and very long 2-4 fingers. There's no way I could play everything with "holding a bubble" hand shape. My fingers go near the fallboard a lot, otherwise my thumb wouldn't be where it should be. I also have a bent in my thumb that I can't straighten. So if I curve my fingers a lot without lowering my wrist or letting the hand go flat, my thumb is in the air way above the keys.

Maybe just let your fingers go straighter and learn to play between the black keys. Just make sure the fingers are firm and you use the muscles in your palm to play. Your wrist sometimes has to go higher too. With a long hand and fingers to play with really curved fingers often causes low wrists and that's not something you would want to do continuously, it may cause problems like carpal tunnel. That's my experience, but a good teacher would be more qualified to assist you in finding the right hand position. You shouldn't need to twist your wrist to play the thumb. When I started playing I did that because I tried to make my hand shape as in books. My teacher quickly made me stop, and I needed to find other ways to adjust my hand to the keyboard. For me it would be ideal to have a keyboard with longer and slimmer keys...but the keys are made to an average hand I guess...

I also sit quite far away because I have long arms. If you look at professional pianists many have their own way to sit. I think some of them must have found the textbook way not working...

I just try to play it by ear then ( no pun intended) and try to figure something out. Perhaps straighten out the fingers and thumb a little. There is no way I could ever play between the black keys. My fingers are way too fat. As a matter of fact even while playing the fat part of a white key I have to watch or I'll press a neighbor with it. I wish it were possible to have a teacher, but I am on a shoestring budget being disabled.

For as far as I understand your problem, when you want to play the thumb the next note, you pivot around your finger so that the thumb will hang over the key. Then when you drop your thumb, it pivots and turns your hand towards the next position. Playing the next note, your thumb is hanging loose again. That's good.

Can you post a video of you playing? That would help tremendously. The "holding a bubble" thing is really a bad instruction for most people. If you let your arms hang at your side while standing up, and then bend your arm at the wrist while leaving the fingers in the same shape, that is the shape you want to use at the piano - mostly. It also depends on what piece you are playing. Some pieces and key signatures demand a flatter finger, and some passages demand more curved fingers.

Sounds like a motion-coordination issue. I'm guessing the in/out motion isn't there, if your thumb is slipping and/or your fingers are too far into the black key area at times. But there may be other issues as well.

I am troubled by that statement.

I am far from an expert but I am not sure it is possible to play the piano without ever playing between the black keys. Is there such a thing as a keyboard made especially for someone with very "FAT" fingers ?

You have to be able to play in between the black keys? I suppose maybe if I jammed them in there and turned it sideways I could. I'll have to go to the keyboard and see and make sure I'm not exaggerating but I'm pretty sure that I can't play in between the keys.

On a happier note, I was able to figure out what I was doing wrong and correct the issue thanks to a YouTube video for the business of the Pianoworld owner. If I take my hand and turn it at a 90° angle at the wrist with all fingers and thumb pointing straight down and bring it down at the keyboard where the white and black keys meet and then just naturally let the fingers fall into place and form an arch I'm all set. For some reason I can't form the bubble first before putting my hand to the keyboard. I guess it's just something that will come in time with muscle memory.

I'll post a follow-up on whether or not I can play in between the keys.

Okay that's a load off my mind. As long as I keep the fingers ever so slightly arched I can get them in there. The middle finger is a very very tight and when struck loudly the black keys on each side do go down ever so slightly but they do not sound. It would be difficult to play very fast running pieces in between the keys with the middle finger but I suppose it would be doable.