The Fillet Knife
Fillet knives, or boning knives, are thin and somewhat flexible and are used to cut meat from bone. The flexibility comes in handy as it allows the knife to bend around curves in the bone rather than trying to simply cut through them.
The filet knife shown here is larger-than-average, most filet knives are only around 5 1/2 in. to 6 1/2 in.
You can also find stiffer fillet knives used to remove the bones from pork and beef where the more flexible one is used for fish and poultry.
Another style of fillet knife is similar in flexibility but has a curved blade, as with many styles of knives, it comes down to a personal preference which one to use.