I cannot stress this enough: When shopping for brisket, go for the highest grade you can find, and hand pick the slab with the most fat striation visible. If it is not labeled, chances are it is select. Avoid it. Brisket is the classic example of "garbage in garbage out". Please don't write to me and say you can't figure out why your brisket was tough if you did not buy USDA Choice or better.
Slow Cooker Beef Brisket French Dip Sandwiches