1876
At the Massasoit convention, the first rules for American football were written. Walter Camp, who would become known as the father of American football, first became involved with the game.

1898
A touchdown was changed from four points to five.

1904
A field goal was changed from five points to four.

1906
The forward pass was legalized. The first authenticated pass completion in a pro game came on October 27, when George (Peggy) Parratt of Massillon threw a completion to Dan (Bullet) Riley in a victory over a combined Benwood-Moundsville team.

1909
A field goal dropped from four points to three.

1912
A touchdown was increased from five points to six.

1933
The NFL, which long had followed the rules of college football, made a number of significant changes from the college game for the first time and began to develop rules serving its needs and the style of play it preferred. The innovations from the 1932 championship game-inbounds line or hashmarks and goal posts on the goal lines-were adopted. Also the forward pass was legalized from anywhere behind the line of scrimmage.

1941
The league by-laws were revised to provide for playoffs in case there were ties in division races, and sudden-death overtimes in case a playoff game was tied after four quarters. An official NFL Record Manual was published for the first time.

1945
The inbounds lines or hashmarks were moved from 15 yards away from the sidelines to nearer the center of the field-20 yards from the sidelines.

1946
Free substitution was withdrawn and substitutions were limited to no more than three men at a time. Forward passes were made automatically incomplete upon striking the goal posts.

1948
Plastic helmets were prohibited. A flexible artificial tee was permitted at the kickoff.

1949
Free substitution was adopted for one year.

1950
Unlimited free substitution was restored, opening the way for the era of two platoons and specialization in pro football.

1951
The Pro Bowl game, dormant since 1942, was revived under a new format matching the all-stars of each conference at the Los Angeles Memorial Coliseum. The American Conference defeated the National Conference 28-27, January 14. A rule was passed that no tackle, guard, or center would be eligible to catch a forward pass.

1955
The sudden-death overtime rule was used for the first time in a pre-season game between the Rams and Giants at Portland, Oregon, August 28. The Rams won 23-17 three minutes into overtime.

A rule change declared the ball dead immediately if the ball carrier touched the ground with any part of his body except his hands or feet while in the grasp of an opponent.

1956
Grabbing an opponent's facemask (other than the ball carrier) was made illegal.
Using radio receivers to communicate with players on the field was prohibited.
A natural leather ball with white end stripes replaced the white ball with black stripes for night games.

1960
The AFL adopted the two-point option on points after touchdown.

1962
Both leagues [NFL & AFL] prohibited grabbing any player's facemask. The AFL voted to make the scoreboard clock the official timer of the game.

1966
Goal posts offset from the goal line, painted bright yellow, and with uprights 20 feet above the cross-bar were made standard in the NFL.

1967
The "sling-shot" goal post and a six-foot-wide border around the field were made standard in the NFL.

1969
The AFL established a playoff format for the 1969 season, with the winner in one division playing the runner-up in the other.

1970
The merged 26-team league [NFL] adopted rules changes putting names on the backs of players' jerseys, making a point after touchdown worth only one point, and making the scoreboard clock the official timing device of the game.

1972
The inbounds lines or hashmarks were moved nearer the center of the field, 23 yards, 1 foot, 9 inches from the sidelines.

The method of determining won-lost percentage in standings changed. Tie games, previously not counted in the standings, were made equal to a half-game won and a half-game lost.

1973
A jersey numbering system was adopted, April 5: 1-19 for quarterbacks and specialists, 20-49 for running backs and defensive backs, 50-59 for centers and linebackers, 60-79 for defensive linemen and interior offensive linemen other than centers, and 80-89 for wide receivers and tight ends. Players who had been in the NFL in 1972 could continue to use old numbers.

1974
Sweeping rules changes were adopted to add action and tempo to games: one sudden-death overtime period was added for preseason and regular-season games; the goal posts were moved from the goal line to the end lines; kickoffs were moved from the 40- to the 35-yard line; after missed field goals from beyond the 20, the ball was to be returned to the line of scrimmage; restrictions were placed on members of the punting team to open up return possibilities; roll-blocking and cutting of wide receivers was eliminated; the extent of downfield contact a defender could have with an eligible receiver was restricted; the penalties for offensive holding, illegal use of the hands, and tripping were reduced from 15 to 10 yards; wide receivers blocking back toward the ball within three yards of the line of scrimmage were prevented from blocking below the waist.

1976
Owners adopted the use of two 30-second clocks for all games, visible to both players and fans to note the official time between the ready-for-play signal and snap of the ball.

1977
A 16-game regular season, 4-game preseason was adopted to begin in 1978.

A second wild-card team was adopted for the playoffs beginning in 1978, with the wild-card teams to play each other and the winners advancing to a round of eight postseason series.

Rule changes were adopted to open up the passing game and to cut down on injuries.

Defenders were permitted to make contact with eligible receivers only once; the head slap was outlawed; offensive linemen were prohibited from thrusting their hands to an opponent's neck, face, or head; and wide receivers were prohibited from clipping, even in the legal clipping zone.

1978
The NFL continued a trend toward opening up the game. Rules changes permitted a defender to maintain contact with a receiver within five yards of the line of scrimmage, but restricted contact beyond that point. The pass-blocking rule was interpreted to permit the extending of arms and open hands.

1979
NFL rules changes emphasized additional player safety. The changes prohibited players on the receiving team from blocking below the waist during kickoffs, punts, and field-goal attempts; prohibited the wearing of torn or altered equipment and exposed pads that could be hazardous; extended the zone in which there could be no crackback blocks; and instructed officials to quickly whistle a play dead when a quarterback was clearly in the grasp of a tackler.

1980
Rules changes placed greater restrictions on contact in the area of the head, neck, and face.

Under the heading of "personal foul," players were prohibited from directly striking, swinging, or clubbing on the head, neck, or face. Starting in 1980, a penalty could be called for such contact whether or not the initial contact was made below the neck area.

1988
At the NFL annual meeting in Phoenix, Arizona, a 45-second clock was also approved to replace the 30-second clock. For a normal sequence of plays, the interval between plays was changed to 45 seconds from the time the ball is signaled dead until it is snapped on the succeeding play.

1990
The NFL revised its playoff format to include two additional wild-card teams (one per conference).

1994
There is now a 2 point conversion following touchdowns (teams now have the option of passing or running for two points or kicking for one after a TD); the starting point of all kickoffs will be the kicking teams 30 yard line (moved back 5 yards); kickoff tees used can be no more than one inch in height (previously 3 inches); a neutral zone infraction has been clarified (officials are to immediately blow their whistles whenever a defender enters the neutral zone causing the offensive player(s) directly opposite to move, this is considered a penalty on the defense. If there is no immediate reactional movement by the offensive player(s), there is no foul. (The neutral zone is defined as the space the length of the ball between the offense and defense line of scrimmage); all field goals attempted and missed when the spot of the kick is beyond the 20 yard line, the defensive team taking possession will get the ball at the spot of the kick; on any field goal attempted and missed with the spot of the kick is on or inside the 20, the ball will go to the defensive team taking possession at the 20; the 11 players on the receiving team are prohibited from blocking below the waiste during a play in which there is a kickoff, safety kick, punt, field goal attempt or extra point kick with one exception, immediately at the snap on these plays those defenders on the line of scrimmage lined up on or inside the normal tight end position can block low.

1995
The emergency (third) quarterback may now enter the game in just the fourth quarter, regardless if the other two quarterbacks are able to play. This means that if the third string quarterback enters the game, the first and/or second quarterback may re-enter, unlike the past two seasons where the emergency quarterback would only play off the first two were unable to resume play.
A receiver knocked out of bounds by a defensive player can now return to the field to make a play.
Quarterbacks may now receive communication from the bench via a small radio transmitter in their helmets. This proposal was originally run on a test basis last year during the pre-season, but was scrapped.

1996
The five-yard contact rule will be enforced more stringently.
Hits with the helmet or to the head by the defender will be flagged as personal fouls and subject to fines. This is being done to protect the offense, particularly the quarterback.

1997 When a team fakes a punt and throws the ball downfield, pass interference calls on the two outside defenders who are actually trying to block a coverage man from getting downfield and might not even know the ball has been thrown have been eliminated.
No player may remove his helmet while on the playing field. Doing so will result in a 15-yard unsportsmanlike conduct penalty. Exceptions are during timeouts and between quarters. (The NFL has done this in an effort to "reduce taunting and overexuberant celebrations" and also "in the name of safety.")

1998
The coin toss will be called before the coin itself is tossed (this is a mid-season change).
Tinted visors are banned from players' facemasks except for medical need.
A team will be penalized immediately for having twelve players in a huddle even if the 12th player goes straight to the sideline as the huddle breaks.
A defensive player can no longer flinch before the snap to draw movement from an offensive linemen.
Instant Replay was turned down again.

1999
Instant replay returns with a challange system.
Clipping is now illegal around the line of scrimmage just as it is on the rest of the field.

2000
Instant replay renewed with the same rules.
Celebrations limited to one player. Fines will be assessed for celebrations by two or more players.
Off-Field supervisory titles elminated, preventing coaches from changing teams without becoming head coach, or "in cases where it's written into individual contracts."
Off-Field consolidation of the sport's internet presence into NFL.com. Teams would evenly split the proceeds.
Anyone wearing an eligible number (1-49 and 80-89) can play at quarterback without having to check in with the referee.

2001
Instant replay renewed for three years with the same rules.
Fumble recoveries will be awarded at the spot of the recovery, not where the player's momentum carries him.
Protecting the passer will be emphasized even more.
Taunting rules will be tightened, with 15-yard unsportsmanlike-conduct penalties flagged.
Bandannas and stocking caps are out, but skullcaps with the team colors and logos are OK.

2002
A player who touches a pylon remains in-bounds until any part of his body touches the ground out-of-bounds; continuing-action fouls now become dead-ball fouls and will result in the loss of down and distance; any dead-ball penalties by the offense after they have made the line to gain will result in a loss of 15 yards and a new first-and-10 series; the act of batting and stripping the ball from player possession is legalized; the chop-block technique is illegal on kicking plays; it is illegal to hit a quarterback helmet-to-helmet anytime after a change of possession; after a kickoff, the game clock will start when the ball is touched legally in the field of play; the two-minute exception is eliminated; inside of two minutes, the game clock will not stop when the player who originally takes the snap is tackled behind the line of scrimmage.