Korean traditional houses are called “Han-ok.” The structure and design of Korean traditional houses are in harmony with nature and the environment. Natural materials like stone, wood, or clay are mainly used for house building. Windows covered with paper on a wooden frame emphasized the natural beauty of the house. Roofs are made of tiles or thatch, and the shape of a flying bird on tiled roofs and the round and cozy shape of thatched roofs show the Korean spirit of harmony with nature. Doors covered with paper on wooden frames are good for ventilation and illumination. Floors are made of On-dol, heated by fire under the floor. The fire stove is placed in the kitchen next to the largest room. Walls are made of clay, which cools the rooms in summer and keeps them warm in winter. Korean houses also have gardens, which bring nature to the house. Some upper class people had more cultivated and artificial beauty in their gardens, but usually Korean gardens follow the natural style. In average people’s houses, they had a small yard in front of the house. At the back, they kept big pots for preserving sauces and pastes, around which they planted some flowers and kept a garden.