Just to be clear, baby back ribs don’t come from baby hogs. I, and most of the butchers I know, prefer a spare rib to a baby back. The spare rib bones are spaced further apart and have more meat and fat between them. On a hog, the rib cage starts at the spine (top of the pig) and ends at the belly. On either side of the spine, on the outside of the rib, is the loin. This is where pork chops come from. Beneath the loin is where the baby back ribs come from.
To get baby back ribs, the butcher removes the loin (boneless loin chops or boneless loin roast). He cuts the ribs out by cutting the hog at the neck bones (where country style ribs come from) and before the sirloin (right after the rib cage ends). He then cuts the chime bone off (spine) and slices the ribs in half with a bandsaw. The smaller portion, closest to the spine, is the baby back ribs.
The remaining portion is the spare ribs. Some people then cut them into “St Louis Style Ribs.” This is where the butcher removes the sternum bone (which is the fattiest piece, closest to the belly). This is also known as the rib-tip section.