A kotatsu is a low table with a wooden frame covered by a blanket or futon. On top of it is a table top, and beneath it is a heater usually built into the table itself. This table is typically used as a dining table in Japanese homes.
In the 14th century, during the Muromachi period, a charcoal powered hearth or a brick or stone-lined fireplace was present in every Japanese household. This was called an irori. The irori served as a stove during these times, as it was the primary place where people cook and heat their food. Later on, it was enhanced by creating a wooden platform with a blanket on top that would separate cooking from seating and trap heat. This became known as a hori-kotatsu or hori-gotatsu with hori, meaning ditch, ko meaning fire, and tatsu, which means foot warmer.
In the 17th century, during the Edo Period, the hori-kotatsu was changed by creating a sunken irori, where the wooden platform would be placed around it. A blanket was still draped on it to trap heat and to serve as warmers as people sit.
The kotatsu became more portable by the end of the 17th century, upon the popularization of the tatami, a matting material used in traditional Japanese homes. Instead of the irori being dug out for the charcoals, a pot served as replacement. This was simply put on top of the tatami, which makes it a lot easier to place the kotatsu anywhere. This was then called the oki-gotatsu with oki meaning placement.
During the 20th century, the kotatsu was made completely transportable. Electric heating was introduced by attaching the heating below the table top of the wooden frame. Nowadays, the wooden frame can be replaced with other materials such as plastic.
Japanese houses, unlike western ones, aren’t quite as insulated. There is no central heating. While it could be quite possible for the Japanese to have centralized heating like the west, it is quite expensive due to the structure and design of most Japanese homes. Hence, instead of centralized heating, Japanese turned to spot or space heating as a source of warmth. This is when the kotatsu was introduced, to primarily insulate heat in the body at lower costs.
Most people may think that this may not be effective as the kotatsu can only warm the lower part of the body, but this is not entirely true. During the time when the kotatsu was created, its heat could easily go through the bottom of the traditional Japanese style kimonos and trap the heat throughout the body.
Though we normally find its purpose as a dining table to keep families warm while eating, it is not really exclusive to dining. As long as one wants to keep himself warm under the kotatsu, people may do work, play games and other activities while using it. Some people even take naps or sleep under the kotatsu to keep themselves warm despite of the uneven insulation.
The kotatsu table is proven not only to be cost saving, but also quite versatile. Due to the various enhancements made to the kotatsu, families can use the kotatsu not only during the winter to keep them warm. It can also be used during summer by removing the blanket and unplugging the electric heater, taking out the warm functionality of the kotatsu.