Ondol (; , Hangul: 온돌, 溫堗, ) or gudeul (Hangul: 구들, ) in Korean traditional architecture, is underfloor heating that uses direct heat transfer from wood smoke to heat the underside of a thick masonry floor. In modern usage it refers to any type of underfloor heating, or to a hotel or a sleeping room in Korean (as opposed to Western) style. The main components of the traditional ondol are an agungi (firebox or stove) accessible from an adjoining room (typically kitchen or master bedroom), a raised masonry floor underlain by horizontal smoke passages, and a vertical, freestanding chimney on the opposite exterior wall providing a draft. The heated floor, supported by stone piers or baffles to distribute the smoke, is covered by stone slabs, clay and an impervious layer such as oiled paper. History Origin Use of the ondol has been found at archaeological sites in present-day North Korea. A Neolithic Age archaeological site, circa 5000 BC, discovered in Unggi, Hamgyeongbuk-do, in present-day North Korea, shows a clear vestige of gudeul in the excavated dwelling (). Early ondols began as gudeul that provided the heating for a home and for cooking. When a fire was lit in the furnace to cook rice for dinner, the flame would extend horizontally because the flue entry was beside the furnace. This arrangement was essential, as it would not allow the smoke to travel upward, which would cause the flame to go out too soon. As the flame would pass through the flue entrance, it would be guided through the network of passages with the smoke. Entire rooms would be built on the furnace flue to create ondol floored rooms. Etymology The term gudeul is a native Korean word. According to a Korean folkloric historian Son Jintae (1900 - missing during the 1950-53 Korean War), gudeul originated from guun-dol (Korean), which means "heated stone", and its pronunciation has changed into gudol or gudul, and again into gudeul. The term ondol is Sino-Korean and was introduced around the end of the 19th century. Alternate names include (janggaeng (), hwagaeng (), nandol (), and yeondol ()). Use Ondol had traditionally been used as a living space for sitting, eating, sleeping and other pastimes in most Korean homes before the 1960s. Koreans are accustomed to sitting and sleeping on the floor, and working and eating at low tables instead of raised tables with chairs. The furnace