The Mattress fits the bottom of the box when folded out and gives children a more comfortable sleeping area. When not in use, the mattress can be folded and also be used as a pillow for the wooden bench. The matrass is compatible with our lap safety belt and up to two y-belts. The mattress is made of heavy duty 400 denier colourfastness polyester. It is UV resistant and has a PU coating giving a durable water-repellent and flexible fabric.