A prime rib, or rib roast, is obtained from the rib primal and is located between the chuck and loin primals. Prime ribs are cut from the seven bone section of the rib primal from ribs number six and twelve. Our small boneless prime ribs are about five pounds and are the length of three ribs. Our large prime rib has the length of four ribs.
A prime rib has three major muscles. The longissimus dorsi, or large center eye, the complexus, a smaller side muscle which is not always present depending on which part of the primal the roast is cut, and the spinalis dorsi, also called the cap of ribyeye or deckle. The muscles of the ribeye are held together with tender sinew with large swaths of rich fat between them.
Prime ribs and ribeye steaks are cut from the same subprimal group of muscles. The different preparation methods account for the subtle taste difference. In a pinch, a prime rib can be cut into a plate of beautiful ribeye steaks.