The Division Series (also known as the League Division Series) is the quarterfinal round of the Major League Baseball playoffs. As with any quarterfinal in a best-of tournament, a total of four series are played in this round, two each for both the American League and the National League.
The first use of the term "Division Series" dates from 1981, when due to a mid-season players' strike, that season was divided into two halves, with the winners of each half from each division playing one another in a best-of-five series to decide which team would represent that division in the League Championship Series (this format being common in minor-league baseball). But because the two halves of the season were independent of one another, the winner of the first half had no real incentive to try to win the second half as well (since, unlike in the minor leagues, if the same team did win both halves it was not given a bye into the next round), and a team that won neither half could have actually had the best overall record in the division; indeed, the latter actually occurred, as the Cincinnati Reds and the St. Louis Cardinals had the two best won-lost records (in both halves of the season combined) in the National League (and at the time were in separate divisions, the Reds in the West and the Cardinals in the East; both now reside in the NL Central), with the Reds having the best overall winning percentage in all of Major League Baseball, yet neither advanced to the playoffs because they finished second in their divisions in each half. Until 2012, when the rules changed, this was the only Division Series which actually consisted of teams from the same division playing each other. This particular Division Series was meant as a one-off series due to the strike.
In 1993, the owners approved to institute the Division Series, this time on a permanent basis, given the fact that three of the four series in the one-off 1981 Division Series went to a game 5. Originally the format called for the top team in the AL East to play the 2nd place team on the AL West and vice versa. In 1994, both the National League and the American League realigned, with the number of divisions in both increasing from two to three (with fewer teams in each). At the same time, the number of teams qualifying for baseball's postseason was doubled, from four to eight; henceforth the three first-place teams from each league's divisions would reach the postseason, along with one wild card team from each league (the latter being the second-place finisher with the best regular-season record). However, this expanded playoff format would not return until the following year, because a players' strike, which began on August 12, 1994, led to the cancellation of that season's playoffs and World Series (and caused the 1995 regular season to have 18 fewer games for each team than normal).
Throughout its existence, the Division Series has been best-of-five; however, both the method of awarding home-field advantage in the series and which games the team getting the advantage would host were changed in 1998.
Originally, the Eastern, Central and Western Division champions rotated home-site priority, with the two of them getting the extra home game and the third one and the wild card not. The 2-3 format was used in which the team with the disadvantage hosted the first two games and the team with the advantage hosting the remaining game(s). This made it impossible for the disadvantaged team to clinch the series at home. A similar format had also been used for the League Championship Series from 1969-1984. It also allowed the disadvantaged teams the unusual luxury of starting a series at home, possibly having home field advantage in a three-game series, and a guarantee that they play the maximum of 2 games at home.
This format was changed in 1998 to a 2-2-1 format. Also changed in 1998 was who played who in the division series rather than a rotation deciding. This was needed because there was a 39.5% chance each year that at least one of the leagues would have their top two division champions meet in the Division Series, allowing the worst division champion to have the "advantage" of playing the wild card.
The two division champions with the best regular-season records have been accorded with the home field advantage. Also, the format changed to a 2-2-1 layout with the team having home-field advantage hosting the first, second and (if necessary) fifth games; with this newer format, both teams have the home-field advantage in a sense. While one team gets to host three games (including the critical first and last game), the other team does get two chances out of three (games 3 and 4) of winning the series on its home field. In both the AL and the NL the three division champions are automatically given the top three seeds, seeded 1-3 based on record, and the wildcard is given the 4th seed regardless of record. In both the AL and the NL, the 1 seed played the 4 seed and the 2 seed played the 3 seed, unless the 1 and 4 seed are in the same division. Because teams from the same division cannot play each other in the first round, if the normal matchups would cause this, then the 1 seed played the 3 seed, and the 2 seed played the 4 seed. In all cases the 1 and 2 seed have home field advantage. From 2007-2011, the 1 seed of the league that wins the All-Star game is given another advantage, being allowed to choose their schedule for the series. They can either choose to have an extra day off (usually between games 1 and 2) during the division series and start a day early, or start a day late, with one less off day (only having travel days off, between games 2 and 3, and if necessary 4 and 5). The American League's best record received this option from 2007-2009 and the National League's best record received it in 2010.
A revised playoff system was utilized beginning with the 2012 season, which added a second wild card team for each league. The two wild card teams in each league play a one game playoff to advance. The winner of this game automatically faces the number 1 seed in the league in the Division Series, regardless of whether or not they are in the same division. Due to scheduling conflicts, the 2012 Division Series featured the lower seed hosting the first two games, with the higher seed hosting the next three games. Since 2013, the Division Series uses the 2-2-1 format previously used.
There has been some criticism on how Major League Baseball schedules Division Series games. Teams with large national fan followings like the New York Yankees are almost always scheduled to play in prime time at 8 p.m. ET/5 p.m. PT to generate the highest TV ratings. As a result, West Coast teams generally have to play on the road in the afternoon, when many of their fans are unable to watch the game because they are at work or school. Conversely, when games on the West Coast are played at 10 p.m. ET/7 p.m. PT, many fans on the East Coast are unable to watch a game in its entirety, due to work or school the next day and games ending around 1 a.m. ET/10 p.m. PT, while most West Coast fans are able to watch the entire game as it will not end as late on the West Coast.
However, during the inaugural and 1995 seasons, the Division and League Championship Series was aired by the league's television operation, The Baseball Network, a syndication package between ABC and NBC (as aforementioned, the 1994 LDS did not air at all due to the strike). In order to increase viewership, all games were played in primetime at consistent times, and each affiliate of the network carrying the series could only air one of the games each night, determined by the station's area. While this prevented the issue of afternoon games (and did, as planned, increase viewership), the plan drew ire from critics for not allowing viewers to choose the games they want to watch during the post-season.