The bone-in ribeye steak is cut from the rib primal which covers ribs six through twelve and is located between the chuck and loin primals.
A full ribeye has three major muscles.The longissimus dorsi, or large center eye, the complexus, a smaller side muscle which is not always present depending on where the steak is cut, and the spinalis dorsi, also called the cap of ribyeye or deckle. The cap is universally known as the single most flavorful and tender cut. The muscles of the ribeye are held together with tender sinew with large swaths of rich fat between them
Ribeye steaks and prime rib both come from the exact same subprimal. Think of it as either a very large steak or a smaller prime rib - perfect as a holiday meal for two.