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A doubleheader is a set of two baseball games played between the same two teams on the same day in front of the same crowd. In addition, the term is often used unofficially to refer to a pair of games played by a team in a single day, but in front of different crowds and not in immediate succession.
In Major League Baseball, for many decades, doubleheaders were routinely scheduled several times each season. However, today a doubleheader is generally the result of a prior game between the same two teams being postponed due to inclement weather or other factors. Most often the game is rescheduled for a day on which the two teams play each other again. Often it is within the same series, but in some cases, may be weeks or months after the original date. On rare occasions, the last game between two teams in that particular city is rained out, and a doubleheader may be scheduled at the other team's home park to replace the missed game.
Currently, major league teams playing two games in a day usually play a "day-night doubleheader," in which the stadium is emptied of spectators and a separate admission is required for the second game. However, such games are officially regarded as two single games on the same date, rather than as a doubleheader. True doubleheaders are less commonly played, and usually are of the twi-night variety. Classic doubleheaders, also known as day doubleheaders, were more common in the past, but although they are vanishingly rare in the major leagues, they still are played at the minor league and college levels.
In 1959, at least one league played a quarter of their games as classic doubleheaders, which declined to 10% in 1979 and further to the point that there were eight years between the last two scheduled official doubleheaders. Reasons for the decline include clubs' desire to maximize revenue, longer duration of games, five day pitching rotation as opposed to four day rotation, time management of relievers and catchers, and lack of consensus amongst players.
In a twi-night doubleheader (short for "twilight-night" doubleheader), the first game is played in the late afternoon; after the first game ends, there is a break of 20 to 30 minutes, after which the second game is played. A spectator may attend both games by purchasing a single ticket. Under the Collective Bargaining Agreement, this is allowed provided the start time of the first game is no later than 5 PM. For statistical purposes, the attendance is counted only for the second game, with the first game's attendance recorded as zero. This type of doubleheader is more common in Minor League Baseball as the result of rainouts. They are also played in Major League cities with open-air ballparks and climates that are too hot for day games.
The "classic" doubleheader is like the twi-night doubleheader except the first game is played in the early afternoon and the second in the late afternoon. This was often done out of necessity in the years before many ballparks had lights; often if either game went into extra innings the second game was eventually called due to darkness. However, it is presently less common in the major leagues, even for rain makeups, since the use of lights in baseball stadiums allow most games to be scheduled for the night. Like the twi-night doubleheader, this type of doubleheader is more prominent in the Minor Leagues. The last one scheduled in Major League Baseball was the Pittsburgh Pirates at the New York Yankees on May 18, 2014. Prior traditional doubleheaders were the San Diego Padres at the Cleveland Indians on April 9, 2014, the New York Yankees at the Cleveland Indians on May 13, 2013, the Los Angeles Angels at the Oakland Athletics on July 17, 2011, and the San Diego Padres at the Philadelphia Phillies on August 2, 2003.
In a "day-night doubleheader," the first game is played in the early afternoon and the second is played at night; in this scenario, spectators must buy separate tickets to gain admittance to both games. Except in special circumstances by the approval of the MLBPA, such as a makeup game resulting from a rain-out, this is prohibited under the terms of the 2002 Collective Bargaining Agreement (CBA). On August 22, 2012, the Miami Marlins played a day-night doubleheader at the Arizona Diamondbacks due to a scheduling error violating another section of the CBA, which prohibits 23 consecutive games without a day off. The Elias Sports Bureau does not include this as a doubleheader for the sake of record books, nor do the official playing rules recognize such games as official doubleheaders. However, they are favored by MLB clubs because they can realize revenue from gate receipts for two games.
Since the 2012 season, the CBA has allowed teams to expand their active roster to 26 players for day-night doubleheaders, as long as those doubleheaders were scheduled with at least 48 hours' notice.
Technically, MLB's 2014 season began with a day-night doubleheader on March 22, 2014 between the LA Dodgers and Arizona Diamondbacks. These games were played in Australia on Saturday evening, March 22, and Sunday afternoon, March 23, local time, but due to a +14 hour time difference started at 4 AM Eastern and 10 PM Eastern on March 22, 2014, respectively at MLB Headquarters in New York City. As of July 31, 2014, there are 23 doubleheaders (including opening day) on the 2014 schedule according to ESPN. 
There are three recorded instances of a tripleheader in Major League Baseball, indicating three games between the same two teams on the same day. These occurred between the Brooklyn Bridegrooms and Pittsburgh Innocents on September 1, 1890 (Brooklyn won all three); between the Baltimore Orioles and Louisville Colonels on September 7, 1896 (Baltimore won all three); and between the Pittsburgh Pirates and Cincinnati Reds on October 2, 1920 (Cincinnati won two of the three). Triple headers are now prohibited under the current collective bargaining agreement, except when the first game is the conclusion of a game suspended from a prior date. This would only happen in the extremely rare case of the only remaining dates between teams being doubleheaders and no single games are left for the suspended game to precede.
College and minors
In college and the minor leagues, however, the doubleheader also results in shorter games. In most instances, both ends of such a doubleheader are seven innings, even if it is a playoff game; in 1994, the first game of the five-game Pacific Coast League championship series between Vancouver and Albuquerque was rained out; the two teams played a doubleheader, seven innings each, on the originally-scheduled date of the second game. In the minors, the only exception is when the first game is the completion of a suspended game from a prior day; i.e., the game was started but was halted by weather before becoming an official game. In these cases, the suspended game is played to completion (seven or nine innings, whichever it was scheduled to be when it started), and the second game of the doubleheader is seven innings.
Doubleheaders of note
The home-and-home doubleheader, where each team hosts one game, is extremely rare, as it requires the teams' home ballparks to be in close geographical proximity. During the 20th century and before the advent of interleague play in 1997, only one instance was recorded in Major League Baseball—a Labor Day special event involving the New York Giants and Brooklyn Superbas.
- September 7, 1903
Since interleague play began, the New York Mets and the New York Yankees have on three occasions played home-and-home doubleheaders. Each occasion was due to a rainout during the first series of the season. During the second series of the season, a makeup game was scheduled at the ballpark of the opposing team as part of a day-night doubleheader.
- July 8, 2000
- June 28, 2003
- Game 1: Yankee Stadium (I): Yankees 7, Mets 1
- Game 2: Shea Stadium: Yankees 9, Mets 8 (June 21 makeup)
- June 27, 2008
- Game 1: Yankee Stadium (I): Mets 15, Yankees 6 (May 16 makeup)
- Game 2: Shea Stadium: Yankees 9, Mets 0
On September 13, 1951, the St. Louis Cardinals hosted a double header against two different teams. The first game was a 6-4 win against the New York Giants. The second game resulted in a 2-0 loss to the Boston Braves.
On September 25, 2000, the Cleveland Indians also hosted a doubleheader against two different teams. The September 10 game against the Chicago White Sox in Cleveland had been rained out. With no common days off for the remainder of the season and both teams in a post-season race, the teams agreed to play a day game in Cleveland on the same day that the Indians were to host the Minnesota Twins for a night game. The Indians defeated the White Sox 9-2 in the first game while the Twins defeated the Indians 4-3 in the second.
On July 23, 2013, the Cincinnati Reds and the San Francisco Giants played a unique doubleheader in which the Reds were the designated home team for Game 2 even though the game was held at AT&T Park, the Giants ballpark. Since the last game of a 4 game series in Cincinnati was rained out earlier in the year and both the Giants and the Reds had a game the next day, it was not possible to reschedule the game the day after, so the game spilled over into what should have been a 3 game series in San Francisco. MLB decided to preserve what should have been the Reds' home field advantage by making them the designated home team for that game. San Francisco prevailed in an ugly 5-3 victory.
Incidence of swept doubleheaders
In a paper in American Statistician, Michael Goodman makes the claim that doubleheaders are swept more often than they are split. This has held more or less true: in 2013, 18 out of 25 double headers were swept, but in 2012 only 10 out of 20 were swept. As of July 31, 2014, 11 out of 19 doubleheaders have been swept in 2014.
- Joe Lemire (July 15, 2011). "Angels-A's reviving traditional doubleheader, if only for a day". CNN. Retrieved April 30, 2015.
- Stark, Jayson (November 22, 2011). "How the new CBA changes baseball". ESPN.com. Retrieved November 23, 2011.
- "October 10, 1920 MLB Games". Baseball-Reference.com. Sports Reference. Retrieved August 12, 2013.
- Bock, Hal (2000-06-14). "A subway doubleheader". Seattle Times. Associated Press.
- "September 13th, 1951 MLB Games". Baseball-Reference.com. Sports Reference.
- "September 25th, 2000 MLB Games". Baseball-Reference.com. Sports Reference.
- Goodman, Michael L. (December 1969). "On the Incidence of Swept Double-Headers". American Statistician (American Statistical Association) 23 (5): 15–17. JSTOR 2682179. doi:10.2307/2682179.