"...Tatami Mats are constructed of Japanese Rush Grass with a double layer top for extra protection. The inside is a baked Dried Rice Straw Fill for strength and durability.
"Each mat is sewn with a black fabric border [other manufacturers offer a variety of fabric colors and textures] and bound with twine. A moisture resistant barrier is included in the foundation of the mat."
-- http://www.jlifeinternational.com/HomeDecor/tatami.html
Today, most homes in Japan have modern furniture and so have hardwood or carpeted floors. Some homes have a "Japanese room" with tatami mats.
When we had the privilege of a three month stay in Japan, we lived in an apartment without tatami mats.
The mats are kept clean and in good repair by the use of soft slippers in the home.
At times a room size is described by how many tatami mats fit in it.
