Baby back ribs have a thin membrane on the underside (bony side) of the rack. If not removed, the ribs are tough to cut and won't absorb the flavors of the rub as well. You can remove the membrane yourself or have your butcher do it. Removing it takes a little effort but you’ll get faster at it after a couple tries. If you buy your ribs at Costco, or any place where they are pre-packaged, you’ll need to do the removal yourself. Don’t skip this step!