prime rib roast
A standing rib roast, or prime rib roast, is cut from the back of the upper rib section of the steer, and it usually comprises a total of seven ribs starting from the shoulder (chuck) down the back to the loin. The term "standing" means the bones are included in the roast, which allows the roast to stand by itself.
The chuck end (aka the large end or second cut) is bigger in overall size, but it has a smaller rib eye, with several thick layers of fat interspersed between portions of lean meat. The loin end—called the small end or first cut—is smaller in overall size, but it has a larger rib eye, meaning more meat and less fat.
The "prime" in prime rib roast refers to the cut of the meat—not to be confused with Prime, the grade of beef. Prime is the best USDA grade of beef available, having the most marbling (flecks of fat interspersed in the meat) and therefore the best flavor and tenderness. Because of its expense, most Prime beef ends up in restaurants. The grade below Prime is Choice, the grade most supermarkets carry.
When you ask for a prime rib roast at a supermarket, chances are the counterperson will assume you’re referring only to the cut, not the grade, and you will receive a Choice grade prime rib. The quality of Choice grade beef is still quite good, and since a rib roast is a rather fatty cut to begin with, a Choice grade prime rib will make a fine roast. That said, if you want to splurge on the best, you’ll need to order a prime (grade) prime rib roast, and you may have to seek out a specialty butcher shop or high-end supermarket to find one.
Fat is an important part of this roast—it protects the roast from drying out, bastes it while it cooks, and gives the meat incredible flavor. So, you can trim the roast of excess fat, but leave a thin layer (no more than 1-inch thick).
Meat cooks more quickly and evenly when it starts at room temperature, so let your roast stand on the counter, loosely covered, for approximately 2 hours before you cook it. If you've frozen your rib roast, let it thaw completely in the refrigerator, and don't forget to let it come to room temp before you put it in the oven.