Movement on a loose surface differs greatly from movement on any other surface, and playing on clay frequently requires the ability to glide into the ball throughout the stroke. The ball bounce on clay courts creates a mark on the ground, which might assist establish whether a shot was in or out. This is not so with other surfaces. Clay courts are typically hard, which means that they require skill to play well. If you're not used to the feel of the ball on clay then this game will be difficult for you. However, with practice you can learn how to play on it intelligently. The main advantage of the clay court is its comfort for the players. On grass or hard courts, large areas of your body are in contact with the surface, which can cause pain after a while. However, on clay courts, the skin only touches the surface at certain points during the swing, which reduces wear and tear on legs and joints. Clay courts are found mainly in Europe and South America. There are even some outdoor clay courts in California! The WTA (Women's Tennis Association) has been using indoor clay courts since 1992 when they introduced them at the Houston Open. Before then, women's tennis had no real opportunity to show what it could do on outdoor courts due to health concerns for the players. Indoor clay courts allow for night matches and less exposure to the sun.