Skirt is the diaphragm, the muscle that contracts every time the animal breathes. A lifetime of nonstop work built it with an extremely coarse open grain and a lot of fat stored inside the fibers, which is where its flavor comes from. It is also only about half an inch thick, so it needs the hottest surface you own and two to three minutes a side.
There are two skirts and they are not the same muscle. Outside skirt is the diaphragm proper, sitting on the outside of the ribcage under the plate and covered by a tough membrane — 1 to 2 lb, roughly 20 to 24 inches long and 3 to 4 inches wide, thicker and more even, and the better of the two. Inside skirt is the transversus abdominis off the inner abdominal wall, wider at 5 to 7 inches, thinner and more irregular, and tougher; it is what most supermarket skirt is. A diaphragm never rests. It cycles ten to thirty times a minute for the whole life of the animal, which makes it an oxidative muscle: dark with myoglobin, dense with mitochondria, and carrying fuel as fat stored right in among the fibers.
That workload is why skirt eats the way it does. A continuously active oxidative muscle stores intramuscular fat rather than burning it off, so skirt commonly runs 8–12% fat, at or above a Choice ribeye, and the high myoglobin and mitochondrial content give it the strong, almost mineral flavor people either seek out or avoid. The tradeoff is grain: the fiber bundles are thick enough to see from across the room and run in one direction, so it has to be sliced across at 1/4 inch or thinner. That same open structure means more surface area per ounce and real gaps between bundles, which is why marinade travels further into skirt than into a dense-grained cut like sirloin. Thickness sets the cooking rule — at half an inch the center reaches 130°F in about three minutes on a very hot surface and 150°F not long after, so if the surface is not hot enough to brown the outside inside that window, it never will be.
At half an inch there is no time to build a crust gradually; the surface has to be over 600°F so browning outruns the heat conducting to the center.
HowFull chimney of coals, or cast iron preheated 8–10 minutes. Dry the surface completely. 2–3 minutes a side, pull at 125°F, rest 5 minutes, slice 1/4 inch across the grain.
The loose bundles give marinade somewhere to travel, so seasoning gets past the surface layer in a way it cannot on a tight-grained steak.
HowCitrus, garlic, salt, chile, 2 to 6 hours. Longer with citrus starts to chalk the exterior. Wipe it dry before it touches the fire or the surface moisture steams instead of browning. Same 2–3 minutes a side.
A 24-inch strip cannot sit over the hot zone evenly, so the thin ends overcook while the middle is still raw.
HowCut the strip into 5 to 7 inch lengths before it goes on the heat, which also makes the grain direction obvious when you slice later. Start the thicker pieces first and add the thin ones a minute behind.
The same connective tissue that makes an underheated skirt chewy converts to gelatin above 180°F, and the coarse bundles come apart into strands.
HowSear, then 300°F covered with liquid halfway up, 2 to 2.5 hours to about 200°F internal, then shred. Skirt is thinner than flank, so start checking at two hours.
Skirt is about half an inch thick. On a 400–450°F grill the center passes 130°F in roughly two minutes and 155°F not much later, and browning still has not started, because Maillard reactions do not run at a useful rate until the surface is above about 300°F and really want 500°F-plus to outrun the interior. What comes off is gray through the middle with a pale, wet exterior — cooked past the point where its intramuscular fat rendered out and its fiber bundles contracted, without the crust that was supposed to be the payment for that heat. None of it comes back. The fat that made skirt worth buying is on the grate, and contracted fibers do not relax. This is the one cut where an underpowered grill is worse than no grill at all: a cast iron pan run flat out on a home burner will beat a moderate gas grill every time.
Ask specifically for outside skirt, and ask whether the membrane has been peeled. Outside skirt is 3 to 4 inches wide and clearly thicker; inside skirt is 5 to 7 inches wide and thinner. Inside skirt is perfectly usable and cheaper — just cut it into 5 to 7 inch sections and slice it thinner after cooking. Look for deep red-brown color and visible white fat running between the fiber bundles; pale, uniformly lean skirt has been trimmed too hard and will not taste like much. Skip anything sold pre-tenderized or already cubed as fajita meat, which is usually inside skirt or flap run through a blade tenderizer, and skip case-marinated skirt for the same reason it got marinated. A whole outside skirt is 1 to 2 lb and feeds three or four.
Doneness: Pull at 125°F and no higher. Rest 5 minutes — a long rest hurts a thin steak, because it keeps giving up heat and the crust goes soft. Slice 1/4 inch or thinner across the grain, cutting the strip into short sections first so the grain direction is unambiguous. Braised: 200°F internal, shredding under a fork.
Two different muscles. Outside skirt is the diaphragm itself: 3 to 4 inches wide, thicker, more evenly shaped, better marbled, and more tender. Most of it goes to restaurants and it costs more. Inside skirt is the transversus abdominis from the abdominal wall: 5 to 7 inches wide, thinner and more irregular, and it is what most retail skirt is. Both cook the same way, at the highest heat you can produce for 2–3 minutes a side, but inside skirt needs thinner slicing and gets more out of a marinade.
Skirt, clearly. It is the traditional cut and the reasons are mechanical: more intramuscular fat, 8–12% against flank's 4–6%; a far more open grain that carries marinade past the surface; and a thinness that lets it cook in 2–3 minutes a side over very high heat, which is the only way it works. Flank is a fine substitute if skirt is not available — it is twice as thick, so cook 4–5 minutes a side and slice it thinner to make up the difference in grain.
The heat was too low, the slicing was wrong, or both. At half an inch, skirt needs 600°F-plus for 2–3 minutes a side; on a moderate grill it sails past 150°F before it browns, which renders its fat out and contracts the fibers. Then there is the grain — skirt's bundles are unusually thick, which makes slicing along them the worst possible bite. Cut the cooked steak into 5 to 7 inch lengths, look at which direction the fibers run, and slice perpendicular at 1/4 inch.
Two to six hours for an acidic marinade. Skirt absorbs marinade better than most cuts because its loose bundles give liquid somewhere to go, but citrus and vinegar still work mostly on the outer 1/8 inch, and past about eight hours they leave the surface chalky and gray. If the marinade has pineapple, papaya, or fresh ginger in it, cut that to two hours — those enzymes keep breaking down protein and will turn the exterior to mush. Wipe the steak dry before it hits the fire, because wet meat steams instead of browning.