Apparatus is provided for integrated, single and multiple person vehicle car seats, modular car seat systems, dual person car seats, variable base height and width car seats, car seat-inserts inserted into another car seat, foldable, 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 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 and height space of the vehicle seat onto which it is mounted.