Japanese-style rooms are
covered with tatami mats.

Tatami is made of straw and rush.
The filling is made of hard-packed straw,
and the smooth facing is made of woven rushes.
The long sides of tatami have a cloth edging.

Tatami is a very good material for floors
because it's so humid in Japan.

Tatami is derived from a verb tatamu which means to fold or to pile.
It shows us that the original tatami was used in different way.
The oldest description is seen in Kojiki,
the oldest history book in Japan written in 712.
In the Heian period (794 - 1191) it has been used only by high status people.
When you see the pictures of "hyakunin isshu "(Japanese traditional cards),
you will find emperors or nobles sitting on tatami
which was placed on the wooden floor as isolated pieces.
In the Muromachi period they began to cover the whole floor with tatami.

Tatami are arranged by the following rules:
1: The corners of four tatami mats should not meet at one point
2: There should not be a line that bisects the tatami layout
either vertically or horizontally
(a line like this resembles an image of harakiri )
3: Only one half size tatami should be used per room.