Baseball is played by two teams of nine players each, and a game is divided into nine innings, yet the question of who invented it has no tidy answer. The story that Abner Doubleday invented baseball in Cooperstown in 1839 is a myth, because Doubleday was at the military academy at West Point that year, not in Cooperstown; the Mills Commission, which looked into who invented baseball, based its final decision almost exclusively on the testimony of one man, Abner Graves. Henry Chadwick had disputed the claim that Doubleday invented the game; he said the game descended from English ball games he had played as a boy. The pitcher throws the ball and the batter tries to hit it with the bat, but what we can document is a formal set of baseball rules, which the New York Knickerbocker club published in September 1845.

Those 1845 rules prohibited the old way of getting a runner out by hitting him with the thrown ball, which made it safe to use a harder ball; the Knickerbockers' pitchers were supposed to deliver the ball underhand, and for that reason the overhand pitch did not arrive until 1884. 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 1859 Henry Chadwick wrote the first modern box score, which recorded runs, hits, putouts, assists and errors, and marked pitchers' strikeouts with the letter K. In 1893 the distance from the pitcher to the batter was extended from 55 feet 6 inches to 60 feet 6 inches, which benefited the hitters. After 1968, nicknamed the "Year of the Pitcher," baseball lowered the pitcher's mound from 15 inches to 10 inches and reduced the size of the strike zone.

The four kinds of hits are singles, doubles, triples and home runs. On a home run, the batter goes around all the bases and scores, and so does every runner already on base. Babe Ruth hit 54 home runs in 1920 and 59 in 1921, and he hit a record 60 home runs in 1927, a mark that stood for 34 years. Babe Ruth hit 714 home runs in his major league career, and as of November 2026 his career on-base plus slugging (OPS) of 1.164 ranks first all-time in the American and National leagues. Hank Aaron hit his 715th home run on April 8, 1974, passing Babe Ruth's career mark of 714, and he finished his career with 755 home runs. Cy Young, on the other side of the contest, won 511 games as a pitcher, almost 100 more than any other pitcher in history, as of November 2026. Earned run average (ERA) is the number of earned runs a pitcher allows per nine innings.

Physics explains the mechanisms by which both sides of this contest operate; a spinning baseball curves because of the Magnus force, and it breaks in the direction that the leading edge of the ball is turning. The Magnus force comes from air pressure that is lower on one side of a spinning ball than the other, which pushes the ball at right angles to the wind. An overhand fastball has backspin, so the force pushes it up and it drops less than gravity alone would make it drop; an overhand curveball has topspin and drops more; a fly ball hit on a typical home run path also has backspin, and the upward Magnus force keeps it airborne longer, so it goes farther. Using a pitching machine to shoot spinning baseballs, University of Illinois physicist Alan Nathan determined that the lift coefficient does not depend strongly on speed at fixed values of the spin factor.

One pitch still resists easy explanation: the knuckleball. Tracking data from knuckleball games showed that knuckleball paths are as smooth as those of ordinary pitches, even though the movement on the pitch is erratic and unpredictable. Spin rate is measured in revolutions per minute, and a fastball with a high spin rate crosses the plate a few inches higher than one with a lower spin rate at the same speed. The pitching rubber is 60 feet, 6 inches from home plate, and a pitched baseball loses approximately 10 percent of its speed on the way to the plate: a pitch that leaves the hand at 95 mph crosses the plate at approximately 86 mph. A pitch must cross the strike zone, the area over home plate between the middle of the batter's body and the kneecaps, and the strike zone has changed several times in baseball history; the current version has been used since 1996, as of November 2026; a batter who gets four balls walks to first base.

The atmosphere through which the ball travels also matters. Air drag takes a great deal of distance off a long fly ball: one that carries 400 feet would carry approximately 700 feet if there were no drag. Air in Denver is approximately 82 percent as dense as air at sea level, so the drag and Magnus forces on a baseball at Coors Field are approximately 82 percent of what they are at Fenway Park. You may have heard that humid air is thicker than dry air, but humid air is less dense than dry air, because a water molecule weighs less than an air molecule. Launch angle is the vertical angle at which the ball leaves the bat: under 10 degrees is a ground ball, 10 to 25 a line drive, 25 to 50 a fly ball and over 50 a pop up. Exit velocity is the speed of the baseball as it comes off the bat right after contact, and Statcast calls a batted ball a Barrel when balls hit with that exit velocity and launch angle have led to at least a .500 batting average and 1.500 slugging percentage since 2015.

The torpedo bat has a shape that makes it feel easier to swing, and the ball is more likely to hit the bat's thickest section, leading to a more solid hit; a mechanical engineer who studies bats said the torpedo bat probably does not make hitters hit the ball harder or faster, and balls may even go a bit slower on average, as of November 2026. Rule modifications continue to arrive as well; as of November 2026, a pitch timer limits the time between pitches to 15 seconds with the bases empty and 18 seconds with runners on base, and pitchers are limited to two pickoff attempts or step-offs per plate appearance. As of November 2026, Major League Baseball's bases are 18 inches square, changed from 15 inches square; 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 records measure endurance and longevity rather than skill with the bat. Cal Ripken Jr. passed Lou Gehrig's mark of 2,130 consecutive games on Sept. 6, 1995, and ended his streak on Sept. 20, 1998; his streak of 2,632 straight games, which began on May 30, 1982, is the longest streak by anyone in baseball history, as of November 2026. Nolan Ryan set the modern single-season strikeout record of 383 in 1973, and he struck out 5,714 batters in all, the most in major league history as of November 2026. In 1908 Cy Young threw his third no-hitter at 41 years and three months old, setting a record for the oldest pitcher to throw a no-hitter that stood for 82 years. In 1980 Rickey Henderson broke Ty Cobb's 65-year-old American League record of 96 stolen bases with 100, and in 1982 he stole 130 bases; he stole 1,406 bases in his career, the most in major league history as of November 2026.

Words to know

• Magnus: spin-push force
• backspin: spin upward
• knuckleball: an erratic pitch
• convention: a meeting
• Statcast: tracking system