One of the things that have marked the modern NFL as different from other major professional sports leagues is the apparent parity between its 32 teams. While from time to time, dominant teams have arisen, the league has been cited as one of the few where every team has a realistic chance of winning the championship from year to year. The league's complex labor agreement with its players' union, which mandates a hard salary cap and revenue sharing between its clubs, prevents the richest teams from stockpiling the best players and gives even teams in smaller cities such as Green Bay and New Orleans the opportunity to compete for the Super Bowl. One of the chief architects of this labor agreement was former NFL commissioner Paul Tagliabue, who presided over the league from 1989 to 2006. In addition to providing parity between the clubs, the current labor contract, established in 1993 and renewed in 1998 and 2006, has kept player salaries low—the lowest among the four major league sports in the United States— and has helped make the NFL the only major American professional sports league since 1993 not to suffer any player strike or work stoppage.