Tatami (畳)

Except for the tōriniwa, all interior floors in a traditional machiya are laid out with tatami. 
Tatami are mats made of rice straw wrapped into a cover of woven rush and the long sides being bound with linen tape (called heri). The tape is normally black, but for special rooms or in special buildings, other colours and patterns may be used. They are between 45 to 60 mm thick.

Tatami were originally used as portable floor covers big enough for a man to sleep on them or two to sit on them. When they eventually became the sole means of flooring, their size was adapted to the space interval between two pillars, called a ken. When standardising the mats, one part of Japan used the mats to determine the space between pillars, while the other part of the country adapted the mats to the distance between two pillars and thus subjugating them to the general grid adhering to traditional building measurement. The first approach is the kyō-ma method, which is also used in Kyōto, while the second approach is called inaka-ma method. The norm mat of the inaka-ma system is 6 x 3 shaku, which are 1,818 x 909 mm. Because the distance between pillars is measured from center to center, walls, which intrude into this space, are not being taken into account, resulting in problems at the edges of a room. The inaka-ma system thus needs five additional, smaller standard sizes to compensate for this.

The kyō-ma system, on the other hand, does not need any additional mat sizes. There is only one standard mat measuring 6.3 x 3.15 shaku, or 1,909 x 955 mm. All spaces between the outer pillars are then adapted to the mat, making one ken the equivalent of 6.3 shaku, while in the inaka-ma system, one ken equals 6 shaku.

Despite the great variances that the different systems entail, even in modern Japan, the size of a room is often given in the number of tatami in it. The most common are 3, 4 1/2, 6, 8, 10, and 12 mat rooms, although in a machiya in Kyōto, an 8-mat-room is considered very big, and rooms with more than 8 tatami are normally limited to very big structures such as temples or palaces.

Once a year, preferably when the general inspection of the house structure is done by a master carpenter, the tatami should be taken out to allow them to air. Due to the porosity of the material, dust easily accumulates in them and humidity poses another problem, with the risk of the mats starting to rot. If the mats are not aired regularly, they need to be replaced rather quickly. 

Nevertheless, a traditional Japanese house would be unthinkable without them and the pleasant feeling they give when sitting and walking on them.