From the passage provided, extract the progression of number of games in an NFL season.

From 1920 to 1934, the NFL did not have a set number of games for teams to play, instead setting a minimum. The league mandated a twelve-game regular season for each team beginning in 1935, later shortening this to eleven games in 1937 and ten games in 1943, mainly due to World War II. After the war ended, the number of games returned to eleven games in 1946, and later back to twelve in 1947. The NFL went to a 14-game schedule in 1961, which it retained until switching to a 16-game schedule in 1978.[38] In March 2021, the NFL officially adopted a 17-game schedule after gaining the agreement of the National Football League Players Association (NFLPA).

In the early years of the NFL the league simply mandated a minimum of 12 games played each season from 1920-1934. In 1937 teams played 11 games. In 1943 teams played 10 games due to World War II. By 1947 teams were playing 12 games per season. From 1961 to 1978 teams played 14 games per season before playing 16 games per season until 2021. Each NFL team plays 17 games per season in present day.