The NBA draft is an annual event that has been held since 1947 in which the National Basketball Association (NBA) clubs can pick players who are eligible and want to join the league. Each draft selects sixty players. A player cannot sign with the NBA unless he has been drafted at least once. If a player was born after the conclusion of the previous season but before its start, he cannot be included in that year's draft; instead, he must wait until the next year when it is possible to select him. There are two main methods by which new players are added to NBA rosters. The first is through the draft, the second is via free agency. A third way for younger players to reach the roster is through the trade market. Teams tend to have certain players they want to keep off the trade block. These are usually role players on low-cost contracts who contribute something different from what the team can find elsewhere. For example, a team might have one of the best rebounders in the game but no one to put up big numbers around him. It could look to move him in order to add a bigger name player. A few players are able to secure their rights through free agency. They can then decide where they would like to play and can usually choose between being signed by a single team or joining an existing contract on another club.