In Japan, the room itself is represented by the number of tatami mats on the floor, so tatami mats are a unit of room size or flooring material, and in the case of apartments, it is common to advertise by the number of rooms and the number of tatami mats on the floor. Room size is generally 4 to 8 tatami mats, and if the room is over 8 tatami mats, the room is of a reasonable size. Early tatami were not fixed, but thin tatami that could be folded and stacked when not in use. Flooring was the norm in upper-class homes, and aristocrats would stack tatami on top of each other to make room for their guests. Over time, the use of tatami spread further, first to the floors of small rooms, then to the floors of large rooms and entire houses, and slowly down the social ladder to the homes of the working class in the late 17th century during the Edo period.