On USDA's pork cutout reports you'll see boneless loin lines split into two fabrication tiers: strap-on and strap-off. The difference is one extra step at the fabricator's bench, but the spread between the two carries a substitution mechanic worth reading.
What "the strap" actually is
A pork loin runs along the spine from the last rib back to the hip. The dominant muscle in cross-section is the longissimus dorsi, that's the loin eye, the round muscle a chop is named for. Running alongside it, attached close to the spinous processes, is a smaller secondary muscle called the multifidus dorsi. In trade vernacular it's "the strap" or "back strap." It's long and thin, a couple of inches wide and about as deep as the lean line of the eye next to it. The fat cap sits above both muscles.
When a boneless pork loin is fabricated, the packer has a choice. Leave the strap attached and you get a boneless loin where chops cut across the cross-section show two muscles glued together: the round loin eye plus a thinner crescent of strap meat next to it, with a visible grain seam between them. Peel the strap off and you get a boneless loin where chops are a single clean muscle face, no seam. The peel takes a few seconds per loin and costs a small yield loss (the strap meat is sold separately or rolled into trim).
Why the chop face matters
Premium chop programs spec strap-off by name. A white-tablecloth restaurant slicing chops off a strap-off loin gets a uniform plate presentation, every chop reads as one muscle, the way a beef ribeye reads. The same chop sliced off a strap-on loin shows two muscles with a seam between them, which reads as a lower-grade fabrication even though the actual eating quality is identical. The bottleneck is plate aesthetics, not anything you can taste.
Commodity retail doesn't care about the seam. A grocery chop in a styrofoam tray is bought on price per pound and the consumer doesn't notice the muscle layout. Mass retail chop programs spec strap-on for that reason: the per-pound discount to strap-off is real and the slicer-face premium is invisible at the point of sale.
IMPS reference
USDA's Institutional Meat Purchase Specifications for fresh pork (IMPS Series 400) names both fabrications explicitly. The standard boneless center-cut loin is IMPS 412, and the variants run as 412A (strap-on, cap-on) and 412B (strap-off). Wholesale spec sheets from major packers carry the IMPS code and the strap status as separate line items because they're separately contracted; a packer's IMPS 412A and IMPS 412B are different SKUs with different price sheets. The full IMPS 400 series document is published by USDA AMS and is the authoritative reference for the fabrication standard.
The spread reads as a substitution signal
Strap-off trades at a premium to strap-on. The size of the premium varies by season and by the relative pull from restaurant chop programs vs commodity retail, which is what makes the spread informative. Three readable states:
When both tiers firm together, the whole center-cut complex is tight, chop demand is broad-based across the spectrum from grocery features to restaurant programs.
When strap-off firms but strap-on doesn't, restaurant chop programs are doing the buying. Premium chop fabricators are accumulating cover for foodservice contracts; commodity retail isn't bidding. This is a foodservice-led signal.
When strap-on firms but strap-off doesn't, the opposite: grocery chains are running chop ads aggressive enough to pull commodity center-cut harder than the premium tier. Mass-retail-led. The spread narrows because strap-on rises into strap-off without strap-off moving.
A widening strap-off-to-strap-on spread is a heads-up that restaurant chop programs are getting expensive faster than mass retail, useful for foodservice operators tracking forward cost. A narrowing spread is the reverse: cheap premium spec is signaling restaurant chop demand has cooled.
Practical takeaways for the buyer
If you're a restaurant or foodservice operator specifying chops on a menu, strap-off is the realistic spec, the plate-face premium is what you're paying for. Track strap-off specifically rather than the broader center-cut composite.
If you're a grocery or club-store buyer running chop ads, strap-on is the spec to track and the price you'll actually pay. Watching the strap-off line will mislead you about your own cost.
If you're a price analyst or a grinder buying loins for further processing, the spread between the two tiers is the substitution mechanic to watch. It tells you which channel, premium chop or commodity retail, is doing the buying in any given week, which is information neither tier alone carries.
The Whole Boneless Loin Strap-On spec on USDA reports is the equivalent commodity-tier line for the whole loin (blade end + center cut + sirloin end), not just the center cut. It trades cheaper than CC strap-on because it includes the less-uniform end pieces; the same strap-on vs strap-off logic applies to its center-cut-only sibling.