4–3

In American football, a 4–3 defense is a defensive alignment consisting of four down linemen and three linebackers. It is probably the most commonly used defense in modern American football and especially in the National Football League.

The Giants employed a 6-1-4 basic formation when they shut out the Browns in 1950, but on many plays this became a 4-1-6 in reality, when the ball was snapped, because the ends dropped off the line to afford extraordinary coverage on passes, Steve Owen, My Kind of Football, 1952, p. 183

Early in the history of the NFL, teams stacked the defensive line of scrimmage with seven linemen, typically using a 7-diamond or the 7-2. With the liberalization of the forward passing rules in 1933, the defenses began to evolve along with the offensive changes, and by the later 1930s, the standard defense in the NFL and college was the 6-2. The successes of the T formation and the introduction of free substitution (abolishing the one-platoon system) in the 1940s led to the almost universal adoption by 1950 of the five-man line. There were two versions popular in the NFL. The 5-3 was an older defense that remained popular through the 1940s and early 1950s. But by the late 1940s, Greasy Neale's defense was creating problems for offenses with a five-man line and four-man secondary. Roughly concurrently, Paul Brown had developed a vertical timing offense. The Browns won every championship of the rival All-America Football Conference from its inception in 1946 through 1949. In the first game of the 1950 season NFL Commissioner Bert Bell had the newly admitted Browns play the champion Philadelphia Eagles on a Saturday ahead of the rest of the league's scheduled Sunday games. The Browns handily won the game in Philadelphia 35–10 and showed they were a force to be reckoned with.

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