The story that Abner Doubleday invented baseball in 1839 is a myth: Doubleday was at the military academy at West Point that year, not in Cooperstown. The real rulebook started with clubs. In September 1845, the New York Knickerbocker club published a formal set of baseball rules, and those rules banned the old way of getting a runner out by hitting him with the thrown ball. That change made it safe to use a harder ball. In 1859 Henry Chadwick wrote the first modern box score, which recorded runs, hits, putouts, assists and errors. At the 1857 baseball convention, the clubs set the game at nine innings, instead of ending it when a team reached 21 runs.

Pitching changed, too. The overhand pitch did not arrive until 1884, because the Knickerbockers' pitchers were supposed to deliver the ball underhand. In 1893 the distance from the pitcher to the batter was lengthened from 55 feet 6 inches to 60 feet 6 inches, which helped the hitters. Today the pitching rubber is 60 feet, 6 inches from home plate, and the pitcher stands on the pitcher's mound.

The strike zone has changed several times in baseball history, and the current version has been used since 1996, as of November 2026. It is the area over home plate between the middle of the batter's body and the kneecaps. Umpires enforce the rules and decide whether a pitch is a strike or a ball. A batter who gets four balls walks to first base, and a foul ball counts as a strike unless the batter already has two strikes.

Even who bats has changed. A designated hitter, or DH, bats in place of the pitcher. The American League adopted the designated hitter in 1973, and the National League adopted it as part of the 2022-26 collective bargaining agreement, as of November 2026.

Some rules in force today are recent changes. Since 2023, Major League Baseball's four infielders must stay within the infield boundary when the pitcher is on the rubber, and infielders may not switch sides. A Major League pitcher must face at least three batters or pitch to the end of a half-inning, with exceptions for injuries and illnesses. A pitch timer limits the time between pitches to 15 seconds with the bases empty and 18 with runners on base. Major League Baseball also changed its bases from 15 inches square to 18 inches square. All of these facts are as of November 2026.

Batting average is a batter's hits divided by his times at bat. Slugging percentage is total bases per at-bat: singles plus 2 times doubles plus 3 times triples plus 4 times home runs, divided by at-bats. Exit velocity is the speed of the baseball as it comes off the bat right after contact, and a faster ball gives the defense less time to react.

Records show what players have done within these rules. Cal Ripken Jr.'s streak of 2,632 straight games, which began on May 30, 1982, is the longest by anyone in baseball history, as of November 2026. Nolan Ryan struck out 5,714 batters, and Rickey Henderson stole 1,406 bases, each the most in major league history as of November 2026. Babe Ruth hit 714 home runs in his major league career, and Hank Aaron finished his career with 755.

One group of rules guards young arms. As of November 2026, Little League limits a 9- or 10-year-old pitcher to 75 pitches a day and an 11- or 12-year-old to 85, and the more pitches a young pitcher throws in a day, the more days of rest he or she needs. MLB's Pitch Smart advises players aged 9 to 12 to throw only fastballs and changeups. Shoulder or elbow pain in a young thrower should always be taken seriously: a player should never throw through it.

Words to know

• convention: a meeting
• overhand: from above
• designated: chosen hitter
• Slugging: power at bat
• velocity: speed