A straw mat covered with woven igusa, or rush, which is laid on the floor of traditional homes.
The tatami sample stored inside the time capsule is no more than a small part of an authentic tatami mat, which weighs about 17-30 kg, and is 4.5-6 cm thick. The heavier and thicker the mat, the higher the quality. Size varies from region to region, but in general, tatami mats are about 180 cm long and 90 cm wide.
In Japan, the size of a room is often described in terms of the number of tatami mats it contains. A large room is a senjo-jiki, or "one thousand mats," and a small room, a yonjo-han, or "four-and-a-half mats." The '70s hit song, "Yonjo-han ", was known as "The Song Sung by Lovers Living Together in a Small Room."