A clay court is one of many different types of tennis court. Clay courts are made of crushed shale, stone or brick. The French Open uses clay courts, making it unique among the Grand Slam tournaments. Clay courts are more common in Continental Europe and Latin America than in the United States, Canada or Britain. Two types exist: red clay, the more common variety, and green clay, also known as "rubico", which is a harder surface. Although less expensive to construct than other types of tennis courts, the maintenance costs of clay are high as the surface must be rolled to preserve flatness. The water content must also be balanced; green courts are often sloped in order to allow water run-off.