What are the difference between outside and inside skirt steak?

Per Kenji Alt's Food Lab article on fajitas:

[T]here are really two distinct cuts from the diaphragm of the steer.

The outside skirt steak is from the plate section, below the rib and between the brisket and flank, and usually comes with the membrane still attached, which needs to be trimmed before cooking.

Inside skirt comes from the flank—it's narrower and thinner than the outside skirt, and comes with the membrane removed.

According to the Houston Press, outside skirt stake is extremely rare in the US, as much of it gets exported to Japan due to a quick in the tariffs. The outside cut is tougher, but extremely flavorful. Both are used in similar applications, if you can get them.

Given that any skirt you buy is probably inside skirt, the difference is likely to be moot. In making fajitas, Kenji Alt recommends grilling it over extremely high heat, to get browning flavors on the outside, and not overcook the center. He recommends not cooking it over medium rare.

A couple of pictures:

1. The inside or inner skirts are much tougher, cheaper & wider than outside or outer skirts!! Whoever says the opposite is true is confused. You can drive a truck over inside skirts and they'll still be tough.

2. Every high end restaurant and steakhouse (Gibson's steakhouse in Chicago for example) uses outside skirts because they are more tender. Much more expensive than inner skirts for this reason and because many are exported by packers to the far east.

121D 4  Plate, Inside Skirt          411.15
121C 4  Plate, Outside Skirt (IM)    638.42
121E 6  Outside Skirt, peeled (IM)   943.69

Above are current box beef prices from packers - on Sept. 4, 2015. Notice the difference in price between whole (not portioned) inside & outside skirts.

Having been a meat cutter for 40 some odd years ALWAYS buy outside skirt steak inside is only tender if ground for burgers! Outide currently (2017) at Public is $11 a lb, but still a family favorite!

They are similar in appearance, but have different tastes. In addition to the previous comment, I would like to add that tha outside skirt is a bit chewer, and fattier, than the inside. The inside skirt is pricier, but in my opinion, well worth the price difference. I can get outside skirt about $6 a pound, whereas inside is usually close to $10. You can cook them the same way. The best result I've experienced, is barbecued with only sea salt as a condiment. Once cooked, you are encouraged to put chimichurri sauce on it for added flavor. In my opinion, inside skirt is as, if not better, tasting that even filet mignon!

Outer skirt is much harder to find, and much of what's left in the US is sold to restaurant vendors, in particular, Hispanic restaurants that offer carne asada dishes. If you are able to grab it, it should be treated in the same way as inner, but has a much higher fat content. Outer would be the ribeye, inner would be a sirloin. I'm sure you can make your mind up from there.

I buy outside skirt steaks for my restaurant and they are more flavorful, tender and expensive. They are more forgiving than inside skirts. If you over cook an inside skirt more than med rare it is leather. I have yet to find outside Skirt steaks in grocery stores. As of 6/29/15 21-28 day aged choice certified angus beef outside skirts are going for around $10.25lb cleaned and peeled

I have eaten skirt steak for over 80 years and the outside skirt steak is the best and the most expensive. You can buy it in high end grocery stores. When I was a child my mother soaked it in egg for a time, dipped it in bread crumbs and fryed it in bacon grease. I still make it this way. You can now buy organic skirt steak and in my area this is over $20 a pound. You can sometimes pick up inside skirt steak that is of course cheaper but it should be as narrow as you can find as this seems to taste better

the inside skirt needs a stronger marinate because it taste more like the insides (liver, kidney, etc). it is cheaper.