Abstract: A modular seat back system for a vehicle, including a type-A seat back frame having a center shoulder seatbelt anchorage, and a type-B seat back frame having a smaller width. The left edges of the seat back frames are substantially similar in shape and size, and the right edges of the seat back frames are substantially similar in shape and size. The symmetry between the edges allows the modular seat back system to form a “40/40 configuration” with two type-B seat back frames in smaller vehicles, a “40/60 configuration” with one type-A and one type-B seat back frame in medium-sized vehicles, and a “60/60 configuration” with two type-A seat back frames in larger vehicles. Further, the dual-nature of the center seatbelt anchorage allows the modular seat back system to form a “40/60 configuration” in left-hand-drive versions of a particular vehicle line, and a “60/40 configuration” for the right-hand-drive versions.