Korean traditional architecture is characterized by its harmony with nature. Ancient architects adopted the bracket system characterized by thatched roofs and heated floors called ''ondol''. People of the upper classes built bigger houses with elegantly curved tiled roofs with lifting eaves. Traditional architecture can be seen in the palaces and temples, preserved old houses called ''hanok'', and special sites like Hahoe Folk Village, Yangdong Village of Gyeongju and Korean Folk Village. Traditional architecture may also be seen at the nine UNESCO World Heritage Sites in South Korea.