ok, take the whole loin. lets say its about 2 feet long (or at least it is when it comes into the meat shop) imagine a really really thick tbone steak, like 2 feet thick, covered with fat and stuff. take a filet mignon, which tapers down to an end, and place it on the side of the really long tbone. now, the end where there is little to none tenderloin, those are t-bones. where there is more tenderloin, those are porterhouses. cut into 3/4 inch slices, and you have your respective steaks. rip the whole tenderloin off of the loin and slice it seperately, you have filet mignon.