Kind of carpet made of dry rice straws, the tatami is made of fixed size tiles of 90cm x 180cm (*), and it is therefore also a unit of area.
Although the tatami is often replaced by other flooring in modern appartments, most japanese people still like to have a washitsu (和室, traditional japanese room), which they use as a living room or as a bedroom. The size of washitsu’s is standardized: rooms of 6 or 8 tatami are the most common, but one also sometimes sees tiny rooms of 4.5 tatamis.
Important notice! It is mandatory to take off your shoes to walk on tatami: Japanese people become mad if you forget to do so!
(*) The size of a tatami is not accidental: it represents the size of a lying man…