Two different muscles sell under the name skirt steak, and the market prices them like two different cuts because they are. The outside skirt is IMPS 121C: the diaphragm muscle itself, hanging on the underside of the short plate. The inside skirt is IMPS 121D: the transversus abdominis, a flat muscle off the inner belly wall. Both are thin, heavily grained, marinade-hungry cuts, which is why both live in the fajita trade, and that is where the similarity ends.
What separates the muscles
The outside skirt is the narrower, thicker, more uniform piece, and the one with the beef-fat richness the fajita houses built their reputation on. It usually trades peeled or gets its membrane pulled at the plant, and portioning it is straightforward because the muscle runs even. The inside skirt is wider, thinner, and less regular. It eats well marinated and sliced against the grain, but it cooks less forgivingly and portions less cleanly, which is why it skews toward retail marinated programs and value fajita formats while the outside carries the restaurant spec.
What the market says
Over the last five years the Choice outside skirt has printed above the inside on 98 percent of trading days, with a median premium around 171 cents per pound. That is the price of the restaurant spec: Tex-Mex and fajita chains write the outside skirt into their menus by name, and a menu spec is the stickiest demand there is. The inside competes with every other marinating cut for the flexible business.
Buying against the spread
The practical question is whether the application truly needs 121C. A build-your-own fajita kit at retail reads nearly the same with well-handled inside skirt at a steep discount; a steakhouse skirt entree does not. Both specs print daily on the same USDA report, so the premium is checkable every session, and the demand calendar around Cinco de Mayo, when the fajita complex sees its loudest retail week, moves both skirts together.