A kotatsu is a low table with a heater fixed under its top. A thick quilt is laid over the frame, the tabletop goes back on over the quilt, and everyone sits on the floor with their legs underneath. The room stays cool. The space under the table is warm, and that is where the family spends the winter evening.

It is one of the oldest pieces of furniture still in daily use in Japan, and it rests on an idea that most modern heating has abandoned: warm the people, not the building.

Where it comes from

The kotatsu began in the Muromachi period, around the fifteenth century, as a frame placed over the dying charcoal of the irori, the sunken hearth in the middle of a farmhouse floor. A cloth over the frame held the heat. Later versions sat over a pit cut into the floor, the horigotatsu, so people could sit with their legs hanging down as if at a Western table. The portable kind, a box of charcoal under a frame, could be moved from room to room.

The electric kotatsu arrived after the war and spread through the 1950s and 1960s, when a heating element replaced the charcoal and the pit was no longer needed. The form has barely changed since. Most homes own one, it comes out of the cupboard in November, and it goes back in the spring.

Why it works

Japanese houses were built for summer, with thin walls and sliding doors, and heating the whole of one was never the plan. Heat went where the body was: a brazier by the hands, a hot bath before bed, a quilt over the legs. The kotatsu is the most sociable version of that. A small element of a few hundred watts, trapped under a quilt, keeps four people comfortable in a room that is otherwise cold.

The cost follows from the design. Heating a table is cheaper than heating a room, and much cheaper than heating a house, which is why the kotatsu survived the arrival of central heating and air conditioning in Japan rather than being replaced by them.

There is also a quieter effect. A kotatsu pulls a household into one place. Winter evenings in Japan are spent around it, with mikan, the small mandarins of the season, in a basket on top, and a cat somewhere underneath. The phrase kotatsu de mikan, mikan at the kotatsu, is shorthand for the whole season. The joke that nobody can get out of one once in is only half a joke.

The parts

A kotatsu has three parts. The frame and heater, usually a square table 75 to 105 centimetres across and about 40 high. The kotatsu-buton, the quilt, square and larger than the table so it reaches the floor on all sides. And the top board, which sits loose over the quilt. A rug underneath, often with a layer of insulation, keeps the warmth from going into the floor.

In spring the quilt is washed and stored, the top goes back on the bare frame, and the kotatsu becomes an ordinary low table for the rest of the year. One object, two seasons.

For a home outside Japan

A real kotatsu can be bought outside Japan, and the frame and heater are not expensive. But the idea travels without the furniture.

Warm the person first. Before turning up the thermostat, a blanket over the knees, a hot-water bottle, warmer socks and a hot drink do most of the work at almost no cost. In a kakeibo, heating is the fixed cost that rises most between November and February, and this is the line that moves it.

Make one warm place. A corner with a lamp, a rug and a blanket, where the household gathers after dinner, does what the kotatsu does socially. The rest of the home can be a few degrees cooler if there is one place that is reliably warm.

Bring the season to the table. A bowl of mandarins, or whatever fruit the season has, on the table where people sit. It is a small thing, and it is the part of the kotatsu people remember.

The kotatsu is not a luxury object. It is a cheap, slightly awkward table that has lasted five centuries because it answers a simple question correctly: what actually needs to be warm?

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This article is part of a series exploring Japanese philosophy and intentional living:

• Kanro, Cold Dew: The Japanese Season That Begins in Early October
• Koromogae: The Japanese Seasonal Wardrobe Change
• Kakeibo: A Clear-Eyed Guide to Japan’s Pen-and-Paper Budgeting Practice
• Wabi-Sabi in Winter: The Season That Was Always Its Home