If we imagine a Japanese room, surely the first thing we think about is the characteristic mat that covers the floor, that is the Tatami (畳). They are usually found in the rooms of old houses or in a room of modern houses, stores, tea houses, in the rest rooms of onsen (hot sptring)…
Usually people walk barefooted on the tatami barefoot trying not to dirty it, and if you want to sleep over normally you use a futon.
They are made with a measure that is usually around 90cm x 180cm and about 5cm thick, although the measurement varies depending on location in Japan. It can also be manufactured in halves.
Although in the ancient times tatami were produced completely of straw, now the interior is rice straw and rush outside is interlaced. The edge is protected by a cloth which is usually green or blue, but for example in Okinawa they are made without this edge.
Traditional Japanese architecture uses the measure of a tatami to design the rooms, and it is very important to follow the rules to place them correctly and traditionally try to bring good fortune and ward off bad luck.
Depending on the room, it may have a certain number of mats, and if necessary, some medium-mat. They have never been placed in a grid, and the main rule is to never have more than two points matching the corners of the mat in one place.
One final note, when walking on tatami, try not to step the edges.