Cuts of Beef
The cuts of beef in the butchers shops in Mexico can be slightly different to what I am used to and it can sometimes be confusing to identify exactly where in the animal a particular cut comes from, and also what the best cooking method for a particular cut is.
This article by Karen Hursh Graber gives as good a summary as one could hope for. It has finally cleared up the arrachera question for me, which US writers regard as equivalent to hanger steak, flank steak or skirt steak pretty interchangeably (it's skirt steak). It's a very useful resource and I've used it quite a bit over the past three years. The diagram below is from the same source.
When it comes to cuts of beef, especially the popular Mexican cuts which are destined for the grill, you need to remember that quick fast cooking and medium rare meat doesn't necessarily equate to tender meat.
Arrachera for example, can be quite stringy when cooked medium rare and works much better when cooked through completely. Similarly, aguja nortena, which is very common here in Monterrey for BBQs, works better when grilled slowly and served well done. It seems counter intuitive, but in these cases, well done is more tender. The mesquite coals that are used here help by providing a steady but not fierce heat that allows relatively slow grilling.
Agujas themselves are cut from the underside of the animal, about six inches by three inches with some bone along one of the long edges. These steaks are cut quite thin - to about a centimetre. At parties they are often cooked, a couple for each guest and offered with tortillas. They are best when seasoned simply, just salt and a sprinkling of Worcester sauce before grilling.