Tatami mats with wooden corridors running between make up the traditional flooring of Japanese houses. The mats are made of straw piled lengthwise and crosswise and stitched with hempen thread; the better the quality of the mat, the more heavily it is stitched. Over the rough base is laid a tightly woven rush surface called goza. The edges of the mat are bound with silk, Iinen or cotton according to the quality of the mat, either in plain colours or brocade designs.One mat weighs between 17 and 30kg and is 4.5 to 6cm thick. The size of the mat is measured in shaku (see Weights and Measures) and the standard size varies slightly from district to district; all are about 180cm long by 90cm wide. The size of Japanese rooms is often stated in terms of the number of mats that will cover the floor. Tatami are kept scrupulously clean - aided by the Japanese custom of removing shoes at the door.