Each player uses a racquet that is strung with cord to strike a hollow rubber ball covered with felt over or around a net and into the opponent's court. The object of the game is to play the ball in such a way that the opponent is not able to play a good return.Two exceptions are that from 1908 to 1961 the server had to keep one foot on the ground at all times, and the adoption of the tie-break in the 1970s. A recent addition to professional tennis has been the adoption of electronic review technology coupled with a point challenge system, which allows a player to contest the line call of a point. The comprehensive rules promulgated in 1924 by the International Lawn Tennis Federation, now known as the International Tennis Federation (ITF), have remained largely stable in the ensuing eighty years, the one major change being the addition of the tie-break system designed by James Van Alen. The components of a tennis racquet include a handle, known as the grip, connected to a neck which joins a roughly elliptical frame that holds a matrix of tightly pulled strings.