Apparatus for multiple person vehicle seats, specialized car seats, modular car seat systems, dual person car seats, and overlapping car seats, for use in a land, sea or air vehicle, mounted onto the vehicle's built-in seat and used for securing a car seat user, such as a small child, within a vehicle's regular seating positions. Car seat systems include components enabling addition or removal of seating positions for a desired number of car seat users as needed, limited only by available width space of the vehicle seat onto which it is mounted. Car seat systems and/or multiple person car seats generally provide more and/or specialized seating positions for persons in car seats than provided for regular passengers in the vehicle width space taken up by the car seat systems or the multiple person car seat.