Baseball is played by two teams of nine players each, and a run is scored when a player goes around the bases and gets back to home plate. The story that Abner Doubleday invented the game in 1839 is a myth, because Doubleday was at the military academy at West Point that year, not in Cooperstown. In September 1845, the New York Knickerbocker club published a formal set of baseball rules. Those rules banned the old way of getting a runner out by hitting him with the thrown ball, and that change made it safe to use a harder ball.

More rules came as clubs met to agree on the game. At the 1857 baseball convention, the clubs set the game at nine innings, instead of ending it when a team reached 21 runs. If a game is tied after nine innings, extra innings are played until one team is ahead. In each inning, each team bats once: one team in the top half and the other in the bottom half. Umpires enforce the rules and decide whether a runner is safe or out and whether a pitch is a strike or a ball.

In 1859 Henry Chadwick wrote the first modern box score. It recorded runs, hits, putouts, assists and errors, and it marked pitchers' strikeouts with the letter K. Batting average is a batter's hits divided by his times at bat.

The ball itself is built with care. A Major League baseball has a cork core with two layers of rubber, wool yarn wound around it, and a cover that workers stitch on by hand. A baseball has 108 double stitches, weighs 5 to 5.25 ounces and measures 9 to 9.25 inches around. The pitcher throws it from a rubber that is 60 feet, 6 inches from home plate.

Some rules are in force in Major League Baseball today. As of November 2026, a pitch timer in Major League Baseball limits the time between pitches to 15 seconds with the bases empty and 18 seconds with runners on base. Major League Baseball also changed its bases from 15 inches square to 18 inches square, as of November 2026. The bases are 90 feet (27 meters) apart.

Rules also protect young players. 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. The more pitches a young pitcher throws in a day, the more days of rest he or she needs. Shoulder or elbow pain in a young thrower should always be taken seriously: a player should never throw through it. A pitcher and a catcher throw a lot, so a player should not do both jobs for the same team.

Two records show how much baseball players do. 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, the most in major league history as of November 2026.

Words to know

• convention: a meeting
• putouts: outs made
• innings: parts of play
• cork: light tree bark
• myth: false story