The kotatsu, and the manners of sharing one warm table through a Japanese winter

In a winter sitting room in Nagano or Akita, the first thing you notice is that nobody gets up. Four people sit on the four sides of a low table. A quilt falls from its edges to the tatami, and there is a bowl of mandarins in the middle. Near the window the room is cold enough that a cardigan is not a choice. All the heat is under the table. To join them you lift one corner of the quilt and slide your legs in, and for the next hour you are part of a small arrangement with rules of its own.

A table, a quilt, a heater underneath

The kotatsu (炬燵 / こたつ) is three layers stacked together. At the bottom is a low frame with an electric heater fixed to its underside. Over the frame goes a thick quilt, the kotatsu-buton (こたつ布団), and on top sits a flat board called the tenban (天板), which holds the quilt down and gives you a usable tabletop. In spring the quilt comes off and the kotatsu is an ordinary low table again, so many households keep one all year and only dress it for winter.

An older form is the hori-gotatsu (掘りごたつ), a well cut into the floor so your legs hang down as they would from a chair. It was once warmed by charcoal at the bottom. Before both came the irori (囲炉裏), the open sunken hearth still kept in the thatched farmhouses of Shirakawa-gō (白川郷) and in some mountain minshuku (民宿) guesthouses. All three work the same way. Japanese wooden houses were built for humid summers, and in winter the custom was to warm people at a single point and leave the rest of the room cold.

The manners under the blanket

These manners are rarely written down, but most households follow them. Keep your feet roughly in your own quarter. Legs that drift across the middle end up against someone else's, and the polite response is a quiet apology and a retreat. Lift the quilt only as far as you need to get in or out, because every gap lets the warm air spill out over everyone's knees. Coats, bags and damp outdoor socks stay outside.

The top of the table is shared. A basket of mikan (みかん), the small, easy-peel winter mandarins, is the standard picture for a reason. So are a pot of green tea and a plate of senbei (煎餅) rice crackers. Someone peels a mikan and leaves the skin in a neat curl on the board, and someone else passes the basket along without being asked. The host often takes the side nearest the kitchen, because the host is the one who will keep getting up.

The warning you hear most is not to fall asleep in it. Kotatsu de neru to kaze o hiku, people say: sleep in the kotatsu and you will catch a cold, because your legs bake while your shoulders sit in a cold room. People do it anyway. Kotatsu kara derarenai, I can't get out of the kotatsu, is the standard winter excuse for not answering the door.

Where a traveller actually meets one

In cities, the version you are most likely to sit at is a hori-gotatsu in the private room of an izakaya (居酒屋). Restaurant signs and booking pages list it as horigotatsu-seki (掘りごたつ席). It means you take your shoes off at a raised step and sit at a low table with a well underneath for your legs, usually unheated outside winter. For visitors whose knees do not like kneeling, these rooms are the comfortable middle ground, and they are worth asking for when you book a group dinner.

The heated, quilted kotatsu turns up in winter at some ryokan (旅館) and minshuku in snow country, such as the Tōhoku prefectures of Akita (秋田) and Iwate (岩手) or the mountain valleys of Nagano (長野). When an inn has one, it usually sits in the tatami sitting area rather than where the futon are laid out. Many rooms rely on an air conditioner or a kerosene heater instead, so ask the inn when you book rather than guessing from the photographs.

Kotatsu that move: a boat and a train

Yanagawa (柳川), in Fukuoka Prefecture, is a town of canals. Its flat-bottomed punts, the donkobune (どんこ舟), carry passengers along them while a boatman poles from the stern. In winter some operators run kotatsu-bune (こたつ舟): quilts and a heated kotatsu laid down the middle of the boat. You drift under willows and low stone bridges with your legs warm and your face in the cold air, and when the boatman ducks under the lowest bridges, the passengers duck with him. Afterwards, the local dish to look for is unagi no seiromushi (鰻のせいろ蒸し), eel steamed over rice in a lacquered box.

On the Iwate coast, the Sanriku Railway (三陸鉄道) runs a seasonal kotatsu ressha (こたつ列車) on its Rias Line (リアス線), with kotatsu fitted among the seats and the Pacific outside the window. It runs on selected winter days and generally needs a reservation through the railway. Further north in Aomori, the Tsugaru Railway (津軽鉄道) does something different with its stove train: a pot-bellied stove in the carriage, where passengers can grill dried squid sold on board. It is not a kotatsu, but the idea is the same: heat the place where people gather, not the whole room.

What a cold room explains

After an evening at a kotatsu, the layout of a Japanese winter house makes more sense. The corridor is cold, the toilet seat is heated, the bath is very hot, and the time between them is spent moving quickly from one warm spot to the next. A family's winter evening gathers around the one table: homework, dinner, the television, a nap nobody planned. Some households put a portable gas burner on the tenban and cook a nabe (鍋) hot pot there, with steam rising into the cold air above.

Bringing the kotatsu out used to have a fixed date. By old custom, households opened the kotatsu and the hearth on the first day of the boar, i no hi (亥の日), in the tenth month of the lunar calendar, a day linked to protection from fire. Tea schools still mark robiraki (炉開き), the opening of the sunken hearth in the tea room, around November. Children learn the word from the winter song Yuki (雪), whose best-known line puts the household cat exactly where you would expect.

犬は喜び庭駆け回り、猫はこたつで丸くなる。

Inu wa yorokobi niwa kakemawari, neko wa kotatsu de maruku naru: the dog runs happily around the garden, and the cat curls up in the kotatsu. Most families now just wait for the first properly cold week, but the line still sums up the season.

Before you go

From central Fukuoka, take the Nishitetsu Tenjin Ōmuta Line (天神大牟田線) from Nishitetsu Fukuoka (Tenjin) Station (西鉄福岡（天神）駅) and get off at Nishitetsu Yanagawa (西鉄柳川). Several boat operators work from near the station and the canal landings. Kotatsu boats run in roughly the coldest months, and the boat companies or Yanagawa's city tourism site are where to confirm the season, the fares and whether you need to book. The Sanriku Railway's Rias Line runs along the coast between Kuji (久慈) and Sakari (盛) by way of Miyako (宮古). The railway's own site lists the kotatsu train dates, how to reserve and any meal options.

Elsewhere, the season decides everything. Kotatsu come out as the nights turn cold, from roughly late autumn into early spring, and you are more likely to find them in rural inns and older homes than in city hotels. If someone invites you to sit at one, take your outdoor socks off, keep your knees on your own side and pass the mikan. The mistake to avoid is the one every Japanese household warns about: falling asleep under the quilt after a bath and a big dinner. You wake up thirsty, with hot shins and cold shoulders, after everyone else has gone to bed.