Ribeye

A carcass grade is not an average of the whole animal. Marbling is judged at a single cross-section, and both major systems make that cut through the longissimus, the long muscle running down the back that a ribeye is a slice of. Japan opens the carcass between the 6th and 7th rib. The United States ribs between the 12th and 13th. Neither system re-scores marbling at the loin, the chuck or the round: one face is read, and that verdict stands for the whole animal.

For every other cut, that is a caveat. An A5 tenderloin was never separately assessed for marbling, and the grade on the label is a statement about a face cut somewhere else. For ribeye it runs the other way, and it is the single best reason to order this cut when you want to see what a grade means: you are looking at the same muscle the grader looked at, near enough the same place.

Near enough, not exactly. The two systems cut at opposite ends of it, and the rib primal itself spans several ribs, so a given steak is a neighbour of the assessment face rather than the face itself. Marbling does vary along the muscle. The honest claim is that ribeye is the closest correspondence available between a number on a label and the meat on a plate, not that it is a guarantee.

Sitting on top of the eye is a crescent of a different muscle, the spinalis dorsi, usually called the rib cap. It is more marbled and looser-grained than the eye it wraps, and it is the part many cooks consider the best eating on the animal. Some sellers separate it and sell it alone, which is why a Wagyu rib cap can cost more per ounce than the ribeye it came off.

This matters when comparing prices. A bone-in ribeye, a boneless eye with the cap attached, and a cap sold separately are three different products at three different prices, and a menu saying only "ribeye" has not told you which one is arriving.

Ask for the same two things any Wagyu claim should carry: where it came from and what it graded. A prefecture or a named producer answers the first. A Japanese grade such as A5, a USDA grade, or an Australian marble score answers the second, and none of the three converts into another, so a seller quoting a crosswalk is offering a comparison rather than an equivalence.

Japanese menus may write ribeye as ribu-rosu, which maps to the rib rather than to the American sirloin, and that is worth knowing because the neighbouring name does not behave: Japanese "sirloin" aligns with the U.S. strip loin, not the U.S. sirloin primal. That trap is one cut over, and it is explained on the main cuts guide.