Tatami have been used since the Heian Period(794-1185), when they were laid out only for sitting. From the time of the Muromachi Period(1392-1573), they were used to cover the whole floor. A tatami mat is made of straw bundled in layers which are stitched together, and the surface is covered with tightly woven rushes. One tatami mat measures about 90A~180 cm(about 3’A~6′), and the size of a Japanese room is expressed by its number of tatami mats. It is worthy of note that Japanese customarily avoid stepping on the edge of a tatami mat, because that will hasten its wear and tear.