Tatami ?　What is it?

Tatami mats are a traditional type of Japanese flooring. Made of woven soft rush straw, and tatami are made in individual mats of uniform size and shape, bordered by brocade or plain green cloth.

In Japan, even if a room does not have tatami mats and covered with carpet or wood instead, the size of the room is expressed by how many tatami mats would fit in there.
The traditional dimensions of the mats were fixed at 90 cm by 180 cm (1.62m²). However, mats from Kyoto (Kyo-tatami) are slightly larger than those from Tokyo and eastern Japan at 95.5 cm by 191 cm (1.82m²)

More information　Tatami[wikipedia]
http://en.wikipedia.org/wiki/Tatami