Flat bone sirloin. The Grocer’s Encyclopedia (1911). rawpixel.com / flickr / CC BY 4.0
Cut from the sirloin area of the cow, a flat bone sirloin steak has a flat, cross-cut bone from the hip in it. Occasionally, it may contain a small segment of another bone, which is a vertebra bone from the back.
The cut contains meat from the following muscles: tenderloin, top sirloin, flap and tri-tip.
The steaks cut in the sirloin are, in order from more tender to less tender: pin bone, flat bone, round bone and wedge bone.
If the bone is removed, the steak will be sold as a top sirloin steak.