The tenderloin is an oblong shape spanning two primal cuts: the short loin and the sirloin. The tenderloin sits beneath the ribs, next to the backbone. It has two ends: the butt and the "tail". The smaller, pointed end, the "tail," starts a little past the ribs, growing in thickness until it ends in the "sirloin" primal cut, which is closer to the butt of the cow. This muscle does very little work, so it is the most tender part of the beef.