The 1966 NFL season was the 47th regular season of the National Football League, and the season after which was played Super Bowl I, though it was called the AFL-NFL World Championship Game. The league expanded to 15 teams with the addition of the Atlanta Falcons, thus an odd number of teams (making byes necessary). This was the last season that NFL teams were divided into just two groups called conferences, and only one round of playoffs was played, the NFL Championship game between the two conference champions.
Goal posts were standardized in the NFL. They were to be between 3 to 4 inches (100 mm) in diameter, painted bright yellow, with two non-curved supports offset from the goal line, and uprights 20 feet (6.1 m) above the crossbar. In 1967, the new "slingshot" goal post would be made standard, with one curved support from the ground. In 1974, the goal posts would be returned to the end line, and the uprights would be extended to 30 feet (9.1 m) above the crossbar.
The new goal-post rule is often referred to as the "Don Chandler Rule", referring to Don Chandler, the place-kicker for the Green Bay Packers. Although widely denied, the height increase of the uprights was in reaction to the previous season's Western Conference playoff game in Green Bay. Chandler kicked a controversial field goal that tied the game with under two minutes remaining. The kick was high above the upright, and many spectators thought that the kick missed. Chandler later hit a field goal that defeated the Baltimore Colts in overtime. The Packers went on to defeat the Cleveland Browns in the 1965 NFL championship game.
In the Western Conference, Green Bay's first loss was in Week Five, falling 21–20 in San Francisco to tie them with the Rams. The Rams' lost 35–7 to Minnesota the next week, and Green Bay stayed in front until Week Nine, when Minnesota beat them 20–17. Baltimore's 19–7 win over Atlanta briefly tied it with the Packers at 7–2–0 in Week Ten, but the Colts lost to Detroit the next week, 20–14. The Packers clinched the title in Week Thirteen.
In the Eastern Conference, the St. Louis Cardinals took the early lead, winning their first five games. (The Dallas Cowboys were also unbeaten, but due to a bye in Week One, they had played one fewer game and thus were a half-game behind the Cardinals in the standings). The unbeaten teams met in Week Six, and both were still unbeaten after they played to a 10–10 tie. However, both teams suffered their first defeat the next week, with St. Louis losing at Washington, 26–20, and the Cowboys falling in Cleveland, 30–21. In Week Nine (November 6), St. Louis beat the Giants, 20–17, while Dallas came up short in a 24–23 loss to the Eagles. The next week, Dallas won at Washington 31–30 on a field goal with 0:15 left, while the Cards fell at Pittsburgh, 30–9, cutting their safety margin to a half-game again. St. Louis had a bye in Week Eleven, and a 20–7 Dallas victory over Pittsburgh gave the Cards and Cowboys records of 7–2–1. Both teams won the next week, setting up the stage for their December 4 meeting in Dallas during Week Thirteen. The Cards took a 10–7 lead in the first quarter, but Dallas won 31–17 to take over the conference lead. In Week Fourteen, Dallas hosted Washington, and lost 34–31 on a field goal at 0:08. The Cardinals were in a must-win game against what should have been an easy opponent, the new (2–10–0) Atlanta Falcons. Instead, the Falcons notched their third win and virtually ended St. Louis's hopes to go to the title game. The St. Louis Cardinals, who lost again the next week, never got that close to the Super Bowl again before moving to Phoenix twenty years later.