Tatami (畳) are thick floor mats made from woven rush grass over a firm core, traditionally rice straw. They cover the floor of a washitsu, a Japanese-style room, and they do more than just look and feel good underfoot. In Japan, a room's size is still often given by how many tatami mats fit inside it, not by square meters.
A mat that measures the room
A single standard tatami mat measures roughly 90 by 180 centimeters, and its size sets the scale for the whole room. Say a room is "six mats" or "eight mats," and anyone in Japan pictures roughly how big it is, no ruler needed. The exact size of a mat shifts slightly by region, Kyoto's traditional mats run a bit bigger than Tokyo's, but the basic idea, a room built to fit a neat grid of mats, holds everywhere. Furniture, doorways, and even whole houses have long been planned around this one unit.
Care and comfort
Tatami is woven from igusa, a rush grass, which gives new mats their pale green color and a mild, grassy smell many people find calming. Over the years the surface fades to a warm tan as it's walked on and sits in the sun. Rather than throw a worn tatami mat away, most households simply have the woven surface replaced, a process called omotegae, while the firm base underneath is reused. Bare feet or socks are the rule on tatami, since shoes and even slippers would damage the soft, springy weave.
Words & idioms to take away
Idioms & proverbs to carry away
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一畳 (ichijō): "one mat," the basic counting word for tatami and, by extension, for room size in general.
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藺草 (igusa): the rush grass tatami is woven from, prized for its scent and its gentle give underfoot.