USDA beef quality grades are a federal scoring of expected eating quality, assigned carcass by carcass at the packing plant. There are eight grades on the books: Prime, Choice, Select, Standard, Commercial, Utility, Cutter, and Canner. Only the top three matter to the wholesale and retail markets a buyer trades in; the lower five flow into processing. A separate system, yield grades 1 through 5, scores how much salable meat a carcass produces and has nothing to do with taste.
The grade comes down almost entirely to two things read at the ribbed carcass: marbling, the flecks of fat inside the ribeye muscle, and maturity, the physiological age of the animal. More marbling means a higher grade because intramuscular fat carries flavor and juiciness. Prime requires at least Slightly Abundant marbling; Choice takes Small through Moderate; Select takes Slight. Grading today is done largely by camera systems that image the ribeye and score marbling consistently, with USDA graders overseeing the line.
Two facts about the system surprise people. First, grading is voluntary and the packer pays for it: it is a USDA service, not a requirement, which is why some beef sells ungraded ("no roll") into channels that don't pay for the stamp. Second, the grade is assigned to the whole carcass off one measurement site, the ribeye face between the 12th and 13th ribs, so every cut from a Choice carcass is Choice regardless of how marbled that particular muscle is.
Why Choice versus Select is the money line
Wholesale beef trades with the grade attached: a Choice strip loin and a Select strip loin are different products with different prices, and the gap between them, the Choice/Select spread, is one of the most watched numbers in the industry. The spread widens when steak demand runs hot (buyers pay up for marbling into grilling season and the December holidays) and narrows when demand is soft or when the grading mix runs rich and Choice is abundant. Prime mostly bypasses the commodity spot market, pulled by branded programs, steakhouses, and export before it reaches the open trade.
The grading mix itself is a supply story. Decades of genetics and longer feeding have pushed the share of carcasses grading Choice or better steadily upward, which means Select is a shrinking slice of production; on days when the Select cutout prints close to Choice or even oddly above it on a given cut, thin Select trade is usually the explanation rather than a real demand signal.
Every graded beef price on this site carries its grade explicitly: the cutouts print separately for Choice and Select in the top strip, the cut prices pages show both grade columns side by side, and the charts let you overlay one grade on the other for any cut.