By the 1980s, free agency was in vogue, and with this, owners soon introduced a more modern labor control too: salary caps. Today, the NFL and NHL both have hard caps, which means that no team can exceed a certain spending limit. Under the NBA’s soft cap, teams can exceed the cap, but only to retain their own players — a clause known as the “Larry Bird Exception” as the Boston Celtics used this rule to retain Bird for his entire career. The MLB only has a (very forgiving) luxury tax — a threshold above which teams must pay a certain percentage in penalty for every dollar they spend over the limit. In the 11-year history of the tax, only five teams have ever paid, with the Yankees’ payments constituting 95% of that total.