The baizecloth covering the tables playing surface and rails, usually made from wool or a wool-nylon blend. In use since the 15th century, cloth is traditionally green-colored, chosen for its evocation of grass. Sometimes cloth is improperly referred to as "felt." The properties of the cloth used to cover a table, as well as environmental conditions that can affect it—notably humidity, the degree its been stretched when installed, and its level of cleanness—have a profound effect on play. See also fast.