In the traditional Japanese houses of the last two centuries or so, the floors were all covered with tatami mats (earlier floors were planked or earthen floors, depending on the class), but these days it’s more common to have only one Japanese room where tatami is the norm. Some modern apartments do not have tatami mats. When laid as flooring, the center of the board is not visible, but when viewed from the side or during construction, this area is about 5-6 cm deep and traditionally made of compressed straw or wood chips, but these days it is increasingly being replaced by a layer of polystyrene. The rush cover is stretched over the board, providing an attractive and durable surface over the floor insulation provided by the board itself. The side edges of the tatami mats have colored brocade borders or plain borders, rather than edges, which are called “heli”.