The French Open is played on clay courts.
Roland-Garros is the only Grand Slam tournament still played on clay. The surface is made from several layers, including crushed stone, gravel, limestone and a thin top layer of red clay dust. It is not a solid slab of red earth all the way down, as the popular image sometimes suggests.
Clay slows the ball and produces a higher bounce than grass or hard courts. It also allows players to slide into shots, making long rallies and stamina especially important. Heavy topspin is highly effective because the ball kicks up sharply after bouncing.
A common mix-up is confusing the French Open with Wimbledon, which is played on grass, or the US and Australian Opens, which use hard courts. The tournament was held on other surfaces in its earliest history, but modern Roland-Garros has been associated with red clay for generations.