Tatami

A tatami (畳^?) is a type of mat used as a flooring material in traditional Japanese-style rooms.
The term tatami is derived from the verb tatamu, meaning to fold or pile. This indicates that the early tatami were thin and could be folded up when not used or piled in layers.^[2]

Tatami were originally a luxury item for the nobility.
Tatami were gradually popularized and finally reached the homes of commoners toward the end of the 17th century.^[7] Houses built in Japan today often have very few tatami-floored rooms, if any. Having just one is not uncommon. The rooms having tatami flooring and other such traditional architectural features are referred to as nihonma or washitsu, "Japanese-style rooms".

In Japan, the size of a room is typically measured by the number of tatami mats (-畳 -jō).
Tatami are made in standard sizes, with the length exactly twice the width.
The size of tatami differs between different regions in Japan.