Most fishes fall into one of two shape classifications: round or flat. Salmon, trout, tuna, carp, and bass are examples of the first group, and sole, flounder, and halibut are members of the second. When a round fish is cut into steaks (slices cut perpendicular to the backbone), the pieces from near the center have a slit or missing segment extending up to the backbone, giving the steak a vaguely horseshoe-like appearance. That opening is the wall of the stomach cavity. A steak cut from near the tail has no slit because it was sliced from a section of the fish located behind the abdominal pocket.
A whole fillet (a boneless slice cut lengthwise from heat to tail) of a round or flat fish is tapered, the narrow end being the tail portion.