The diaphragm is a thin, boneless piece of meat from the belly (flank) of the calf. It is a muscle that moves a lot, so as a meat it is quite tough.
It is one of the most flavorful cuts of beef. With a lot of connective tissue, it is an excellent "steak" for grilling.
Both the Inside and Outside Skirt are the cut, boneless, portion of the diaphragm muscle. The Inside Skirt, or inner diaphragm, is a long, flat, fibrous muscle, and is characterized by a deep, meaty flavor.
This cut was not commercialized in America. However, it became popular in Mexico (since it was much more economical) because it was used in the production of fajitas.
This changed in the 1980s, when the fajita dish grew in popularity in America, which inevitably led to an increase in the price of the cut.

