A futon (布団) is the traditional Japanese bed, not a piece of furniture but a set of padded quilts laid directly on the tatami floor at night and folded away each morning. It is bedding you make and unmake every day, and it is the reason a Japanese room can be a bedroom by night and an empty living space by day.
Bedding, not furniture
A proper futon set has two main parts: the shikibuton (敷き布団), a firm padded mattress laid on the floor to sleep on, and the kakebuton (掛け布団), a thick quilt laid over you like a duvet, usually with a pillow often filled with buckwheat hulls. (The Western "futon" sofa-bed is a distant, misnamed relative.) The whole set is soft, thin enough to fold, and light enough to lift, designed to be put away. This is quite different from the Western bed left standing in the room; a futon disappears, and the room it was in becomes something else.
The daily ritual
Living with a futon means a small daily rhythm. In the morning it is folded up and stored in the oshiire (押し入れ), a deep sliding-door closet built for exactly this, freeing the room completely. On fine days futon are draped over balconies and railings across Japan to air in the sun. The warmth and light freshen the padding and kill damp and mites, and the sight of futon hanging out to dry is an everyday feature of a sunny afternoon. Leaving a futon down and unmade all the time has its own dismissive word, mannendoko, a "year-round bed," a small sign of a slovenly household.
Words & idioms to take away
Idioms & proverbs to carry away
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押し入れ (oshiire): the deep built-in closet, closed by sliding fusuma doors, where the futon is folded away by day; a fixture of the traditional bedroom.
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万年床 (mannendoko): literally "ten-thousand-year bedding," a futon that is never folded and put away, left out permanently, a byword for laziness.