The baby back rib (loin rib, back rib, or Canadian back rib) is taken from the top of the rib cage between the spine and the spare ribs, below the loin muscle. The designation "baby" indicates the cuts are from market weight hogs, rather than sows. They have meat between the bones and on top of the bones, and are shorter, curved, and sometimes meatier than spare ribs.
The rack is shorter at one end, due to the natural tapering of a pig's rib cage. The shortest bones are typically only about 3 inches (7.6 cm) and the longest is usually about 6 inches (15 cm), depending on the size of the hog. A pig side has 15 to 16 ribs (depending on the breed), but usually two or three are left on the shoulder when it is separated from the loin. So, a rack of back ribs contains a minimum of 8 ribs (some may be trimmed if damaged), but can include up to 13 ribs, depending on how it has been prepared by the butcher. A typical commercial rack is 10-13 bones. If there are fewer than 10 bones, butchers call them "cheater racks".