If you visited a traditional Japanese house or entered an old room, you probably noticed what you walked on. A floor made from straws with strange connection lines in between. Those mats have already been around for an incredible long time, became a typical factor for Japanese housing and even emerged as the official space measuring unit in Japan: today we talk about Tatami (畳).

Tatami mats are traditionally made using rice straw for the core and a covering of woven soft rush straw for the shell. Tatami are made in standard sizes, with the length exactly twice the width, a ratio of 2:1. Usually, on the long sides, they are wrapped with plain cloth, although some tatami have no edging. The wrapped edges of the Tatami are forming the connection lines that you can see when several tatami mats are used in a room. 

Tatami were gradually popularized and finally reached the homes of commoners toward the end of the 17th century. Before that, Tatami mats were a luxury good, only available to priests, noble people, samurai and aristocrats who were able to afford Tatami in the rooms they intended to sit in.
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, “Japanese-style rooms”.

In Japan, the size of a room is often measured by the number of Tatami mats that fit in it. The measure is about 1.653 square meters for a standard size Tatami. Alternatively, in terms of Japanese area units, room area and especially house floor area is measured in terms of “Tsubo”, where one Tsubo is the area of two tatami mats as a square with around 3.3 meters side length. 
On your next walk, take a look at the estate offers for apartments, the area unit is in Tsubo ! You can do the maths yourself now.