• 1900

A pitcher must win at least fifteen games to qualify as the league leader in the category of Winning Percentage. (The earlier rule stated that a pitcher must appear in twenty-five games.) [10.23]

The shape of home plate is changed, from a twelve-inch square to a five-sided figure seventeen inches wide. [1.05]

• 1901
• 1902

The first two fouls are termed strikes (in the National League). [6.05B]

The catcher is no longer allowed to catch two strikes on a bounce. [4.03]

The infield fly rule is in effect when there are no outs as well as one out. [6.05E]

The American League joins the majors (the National League got started in 1876), and the rule discrepancies begin. For instance, the National League declares that any foul ball not caught on the fly is a strike unless the batter has two strikes on him. The AL does not agree–at least not right away. [6.05]

If an offense is "flagrant" enough, the league president may suspend a player or coach who has been fined and/or ejected by an umpire. [9.05C]

• 1903

If there is only one umpire in a game, he may stand anywhere on the field he likes. [9.03A]

The American League agrees that any foul ball not caught on the fly is a strike unless the batter has two strikes on him. [6.05]

• 1904
• 1905

The height of the pitcher’s mound is established. It may not be higher than fifteen inches above the base lines and home plate. [1.07]

• 1906
• 1907

The umpire gets authority over the groundskeeper. [9.01E]

• 1908

Pitchers are forbidden to scuff or soil a new ball. [8.02]

Four umpires are assigned to the World Series for the first time. Only two work in a game at a given time.

The sacrifice fly rule is adopted, exempting the batter from an at-bat when a run scores after a catch. [10.09]

• 1909

All four umpires assigned to the World Series work in each game.

The pitcher or catcher is charged with an error if a batter reaches first base on a wild pitch or passed ball. [10.13]

A bunt on a third strike is a strikeout. The catcher is credited with the putout. [10.17]

If a runner is thrown out on an attempted double-steal, neither runner shall be credited with a stolen base. [10.08]

• 1910
• 1911

The umpire organization chart is established. The plate umpire–the one who judges balls and strikes–is appointed the umpire-in-chief, and the others are field umpires. An umpire may not interfere with or criticize his colleagues’ decisions. Only the umpire-in-chief may declare the game a forfeit. [9.04]

The captain of a team must notify the umpire-in-chief of any substitution. [3.03]

An umpire must warn players on the bench for excessive yelling before he can fine or otherwise punish them for it. [9.01D]

Before a game begins, the umpire must announce any special ground rules. [9.04]

The cork-center ball is adopted for regular use (it had been used in the previous year for occasional play). [1.09]

• 1912
• 1913

Earned runs are charged to a pitcher when a player scores by means of safe hits, sacrifice hits, bases on balls, hit batters, wild pitches, and balks. [10.18]

• 1914
• 1915
• 1916

In the case of fire, panic, or storm, the umpire does not have to wait until the pitcher has the ball on the mound to call a time-out. [9.04]

• 1917
• 1918
• 1919

Earned runs are also charged to a pitcher when a player scores by means of a stolen base. [10.18]

• 1920

The abolition of the spitball, with a "grandfather clause": each team is allowed to appoint two spitball pitchers for the 1920 season. [8.02]

A ball that hits an umpire is in play. [5.08]

The umpire may suspend play at any time for an accident to a player or an umpire. [4.12]

After a thirty-minute rain delay an umpire may terminate a game. [4 12]

A player must appear in at least one hundred games to qualify as the league leader in Batting Average and Slugging Average. Before this there was no official rule, but it was generally accepted that a man had to play in sixty percent of the scheduled games to qualify. [10.23]

The category of RBI is added to scoring. [10.04]

A runner may not run the bases in reverse order "for the purpose of confusing the fielders or making a travesty of the game." [7.08I]

The ball has its gloss removed before a game by the umpire. [3.01]

Enter the "lively ball." Australian yarn, said to be stronger than its American equivalent, may be wound tighter, so the ball’s bounce and hardness increase. [1.09]

No stolen base is to be credited when the defense makes no attempt to get the runner out. [10.08]

• 1921
• 1922
• 1923
• 1924

Another grandfather clause in the spitball rule: eight National League and nine American League pitchers are officially designated as spitball pitchers and allowed to use the spitter for the rest of their careers. [8.02]

• 1925

Pitchers are allowed to use a rosin bag. [8.02E]

• 1926
• 1927
• 1928
• 1929
• 1930

Pitchers are not credited with a strikeout if a batter reaches first base because of a wild pitch on the third strike. [10.17]

It is a ground-rule double instead of a home run if the ball is hit over the fence in fair territory if the fence is less than 250 feet from home plate. [6.09]

The cushioned cork-center baseball is introduced. [1.09]

The sacrifice fly rule is amended to exempt a batter from an at-bat when a runner advances from first to second or second to third as well as on scoring. [10.09]

• 1931
• 1932

A fair ball that bounces through or over a fence or into the stands is considered a ground-rule double instead of a home run. [6.09]

The sacrifice fly is eliminated. [10.09]

• 1933

Three umpires to a daily game come into regular use.

• 1934

Both major leagues are required to adopt the same brand of baseball. [1.09]

• 1935
• 1936
• 1937
• 1938

The first umpire school opens, in Hot Springs, Arkansas.

The first major league night baseball game is played, in Cincinnati on May 24.

• 1939

Six umpires are appointed to the World Series, two of whom work as alternates.

The pitcher is allowed to have his free foot in front of or behind the rubber, with his pivot foot in front of or on the rubber (but always in contact with it). [8.01]

A batter is credited with a sacrifice fly and not charged with a time at bat if he hits a fly ball that is caught and a runner scores on the catch. This rule lasted only a year. [10.09]

• 1940
• 1941
• 1942
• 1943
• 1944

A batter is no longer credited with a sacrifice fly. [10.09]

The pitcher is permitted to take two steps–one forward, one backward–as long as his pivot foot remains in contact with the rubber at all times. [8.01]

It is a balk if a pitcher throws or fakes a throw to an unoccupied base. [8.02]

The umpire assumes authority over trainers. [9.01E]

• 1945
• 1946

A player must have at least four hundred at-bats to qualify as the league leader in Batting Average or Slugging Average. [10.23]

• 1947
• 1948

Six umpires are hired to work in the World Series, all of them on the field.

• 1949

On December 21 the "new" rules are issued. There are no major changes, but many ambiguities are eliminated, and they are recodified into the ten sections we have today.

• 1950

An umpire may no longer levy fines. That job is reserved for the league president.

The pitcher’s mound must be fifteen inches above the level of the base lines. [1.07]

A player must play in at least two-thirds of his team’s scheduled games to qualify as the league leader in Batting Average or Slugging Average. [10.23]

• 1951

A pitcher must pitch a total of at least one inning for every scheduled game to qualify as the league leader in the categories of Earned Run Average or Fielding Average. (Before this, he had to pitch at least ten complete games and at least one hundred innings.) [10.23]

A player must have at least four hundred at-bats to qualify as the league leader in Batting Average or Slugging Average. However, if there is any player with fewer than the required number of times at bat whose average would be the highest if he were charged with this required at-bat total, then he shall be recognized as the league leader. [10.23]

• 1952
• 1953

Four umpires to a regular game becomes standard practice.

• 1954

A batter is credited with a sacrifice fly and not charged with a time at bat if he hits a fly ball and the runner scores on the catch. [10.09]

Offensive players are required to "carry all gloves and other equipment off the field . . . while their team is at bat." [3.14]

The bat may be made of two or more pieces of wood laminated together. [1.10]

• 1955

The 1951 rule is repealed. A player must have at least four hundred at-bats, period, to qualify as the league leader in Batting Average or Slugging Average. [10.23]

When a base is occupied, a pitcher must deliver the pitch within twenty seconds of receiving it from the catcher. If he fails to do so, the umpire may call a ball. [8.04]

• 1956

A base runner who interferes with a batted ball in order to break up a double play is to be declared out, as is the batter. [7.08H]

• 1957
• 1958

A player must have a total of at least 3.1 plate appearances for every scheduled game to qualify as the league leader in Batting Average or Slugging Average. [10.23]

• 1959
• 1960
• 1961

Minimum fence distances are established for new ballpark construction. [1.04]

• 1962
• 1963
• 1964
• 1965
• 1966

Oversized gloves are banned for use by pitchers, infielders, and outfielders. [1.14 and 1.15]

Batters may apply a grip-improving substance to the bat, though not beyond eighteen inches of its length beginning at the handle. [1.10]

• 1967

» 1967

A player must have a total of at least 3.1 plate appearances for every scheduled game to qualify as the league leader in Batting Average or Slugging Average. However, if there is any player with fewer than the required number of plate appearances whose average would be highest if he were charged with this required number of appearances, then the player shall be recognized as the league leader in Batting or Slugging Average. [10.23]

• 1968

The pitcher’s mound is lowered to ten inches above home plate and the base lines, where it remains today. [1.07]

If a pitcher "goes to his mouth" with men on base, a balk is declared. If the bases are empty, a ball is called. [8.02]

• 1969
• 1970

Runs are earned by a relief pitcher who enters the game in the middle of an inning as if he entered the game at the beginning of the inning. [10.18]

The category of Saves is added to baseball statistics. [10.20]

• 1971
• 1972

All major league players must wear protective helmets at bat. [1.16]

• 1973

The year of the DH. The American League votes to accept the designated hitter rule on a three-year experimental basis. The National League votes against it. [6.10]

A reliever is credited with a save for "protecting" a lead. [10.20]

• 1974

The save rule is amended slightly; no save is to be credited to a pitcher unless the tying run was on base or at the plate or unless he pitched three effective innings. (Before this a reliever was given a save if he maintained the lead, no matter what the score when he arrived.) [10.20]

Umpires may declare illegal pitches without any physical evidence. If they think that the motion of the ball indicates that the pitcher is throwing a spitter or a defaced ball, they may issue a warning and, if it happens a second time, eject a pitcher from the game. [8.02]

• 1975

The ball may be covered with cowhide as well as horsehide. [1.09]

Cupped bats are allowed. [1.10]

The save is refined once more: if the tying run is on deck, a pitcher is credited with a save. [10 20]

• 1976
• 1977

The American League accepts the DH as a permanent part of the rules. The National League reaffirms its opposition. [6.10]

• 1978
• 1979
• 1980
• 1981
• 1982

A pitcher’s ERA is to be calculated henceforth with fractions of innings pitched rather than with full innings. [10.22]

• 1983
• 1984
• 1985
• 1986
• 1987

A pitcher who pitches only a third of an inning in a season will not have his ERA rounded off. His total for innings pitched will be carried as one third. [10.22]

• 1988

Protective helmets are mandatory for catchers. [1.16(d)]

• 1989

The "game-winning RBI" (previously credited to a batter who gave his club "the lead it never relinquished") is eliminated as an official statistic. [10.04(e)]