These big, boneless steaks come from the largest muscle of the sirloin, a continuation of the top loin. Relatively lean, top sirloins vary in tenderness and size, but all make an economical choice for a satisfying steak dinner. Top sirloin sometimes goes by the names sirloin butt and top sirloin butt.
Molly Stevens, author of All About Braising (W. W. Norton, 2004).