Tennis can be played on three different types of courts, which include grass court (grass tennis), hard court and clay court. Also, during a match, the batsman must have two balls in his hand before starting play. This is one of the rules that is not written, but is still taught, because it is more dictated by common sense. Another variation is that if problems occur with the tennis balls during a match, these can be changed at the start of a set or after an agreed-upon odd number of matches. There are essentially two principles of tennis, which are keeping your eyes on the ball and moving during shots (moving or running towards the ball) when the ball is in play. There are two different types of shots a player can make, a ground shot and a volley. A ground shot is when a tennis ball is allowed to bounce once, before being hit. When a ball is hit in the air before bouncing, the shot is called a volley. Players must also serve the ball by throwing it well above their head and touching it in the air, as high as possible. After the ball is in play, the counting begins. Accordingly, points are awarded in series