Drafts are usually permitted under antitrust or restraint of trade laws because they are included in collective bargaining agreements between leagues and labor unions representing players. These agreements generally stipulate that after a certain number of seasons, a player whose contract has expired becomes a free agent and can sign with any team. They also require minimum and sometimes maximum salaries for newly drafted players. Leagues may also allow teams to trade draft picks among each other in exchange for other draft picks or in exchange for players.