In baseball, the designated hitter rule is the common name for Major League Baseball Rule 5.11,  adopted by the American League in 1973. The rule allows teams to have one player, known as the designated hitter (or DH), to bat in place of the pitcher. Since 1973, most collegiate, amateur, and professional leagues have adopted the rule or some variant.  MLB's National League and Nippon Professional Baseball's Central League are the most prominent professional leagues that do not use a designated hitter. 
In Major League Baseball, the designated hitter is a hitter who does not play a position, but instead fills in the batting order for the pitcher. The DH may only be used for the pitcher (and not any other position player), as stated in Rule 5.11. Use of the DH is optional, but must be determined prior to the start of the game. If a team does not begin a game with a DH, the pitcher (or a pinch-hitter) must bat for the entire game.
The designated hitter may be replaced as DH only by a player who has not entered the game. If a pinch hitter bats for, or a pinch runner runs for, the DH, that pinch-hitter or pinch-runner becomes the DH.
The designated hitter can be moved to a fielding position during the game. If the DH is moved to another position, his team forfeits the role of the designated hitter, and the pitcher or another player (the latter possible only in case of a multiple substitution) would bat in the spot of the position player replaced by the former DH. If the designated hitter is moved to pitcher, any subsequent pitcher (or pinch-hitter thereof) would bat should that spot in the batting order come up again (except for a further multiple substitution). Likewise, if a pinch-hitter bats for a non-pitcher, and then remains in the game as the pitcher, the team would forfeit the use of the DH for the remainder of the game, and the player who was DH would become a position player (or exit the game).
Unlike other positions, the DH is "locked" into the batting order. No multiple substitution may be made to alter the batting rotation of the DH. In other words, a double switch involving the DH and a position player is not legal. For example, if the DH is batting fourth and the catcher is batting eighth, the manager cannot replace both players so as to have the new catcher bat fourth and the new DH bat eighth. Once a team loses its DH under any of the scenarios discussed in the previous paragraph, the double switch becomes fully available, and may well be used via necessity, should the former DH be replaced in the lineup.
In Major League Baseball, during interleague play, the application of the DH rule is determined by the identity of the home team, with the rules of the home team's league applying to both teams. If the game is played in an American League park, the designated hitter may be used; in a National League park, the pitcher must bat or else be replaced with a pinch-hitter. On June 12, 1997, San Francisco Giants outfielder Glenallen Hill became the first National League player to DH in a regular-season game, when the Giants met the American League's Texas Rangers at The Ballpark in Arlington in interleague play. 
At first, the DH rule was not applied to the World Series. From 1973 to 1975, all World Series games were played under National League rules, with no DH and pitchers batting. For 1976, it was decided the DH rule would apply to all games in a World Series, regardless of venue, but only in even-numbered years. Cincinnati Reds first baseman Dan Driessen became the first National League player to act as a DH in any capacity (regular season or postseason) when he was listed as the DH in the first game (he was the DH in all four Series games that year). This practice lasted through 1985. Beginning in 1986, the DH rule was used in games played in the stadium of the American League representative.
There was initially no DH in the All-Star Game. Beginning in 1989, the rule was applied only to games played in American League stadiums.  During this era, if the All-Star Game was scheduled for an American League stadium, fans would vote in the DH for the American League's starting lineup, while the National League's manager decided that league's starting DH. Since 2010, the designated hitter has always been used by both teams regardless of where the game is played. 
In June 2010, the Philadelphia Phillies' scheduled 2010 series against the Toronto Blue Jays at Rogers Centre was moved to Philadelphia, because of security concerns for the G-20 Summit. The Blue Jays wore their home white uniforms and batted last as the home team, and the designated hitter was used.  The game was the first occasion of the use of a designated hitter in a National League ballpark in a regular-season game with Ryan Howard being the first player to fill the role. 
In spring training games, the home team chooses whether the designated hitter is used. Occasionally National League teams opt to use the designated hitter, usually when a player is recovering from an injury.
The rationale for the designated hitter rule arose comparatively early in the history of professional baseball. It was observed that, with a few exceptions—most notably Babe Ruth, who began his career as a pitcher with the Boston Red Sox—pitchers are usually selected for the quality of their pitching, not their hitting, and that most pitchers were weak hitters who had to be batted ninth in the batting order and pinch-hit for late in games when their team was trailing. The designated hitter idea was raised by Philadelphia Athletics manager Connie Mack in 1906,  though he was not the first to propose it. The rumors were that he grew weary of watching Eddie Plank and Charles Bender flail at pitches when at bat. Mack's proposal received little support and was even lambasted by the press as "wrong theoretically". The notion did not die. In the late 1920s, National League president John Heydler made a number of attempts to introduce a 10th-man designated hitter as a way to speed up the game, and almost convinced National League clubs to agree to try it during spring training in 1929. 
However, momentum to implement the DH did not pick up until the pitching dominance of the late 1960s. In 1968, Denny McLain won 31 games and Bob Gibson had a 1.12 ERA, while Carl Yastrzemski led the American League in hitting with only a .301 average. After the season, the rules were changed to lower the mound from 15 to 10 inches and change the upper limit of the strike zone from the top of a batter's shoulders to his armpits. In addition, in 1969 spring training, both the American League and National League agreed to try the designated pinch hitter (DPH), but they did not agree on the implementation. Most NL teams chose not to participate. On March 6, 1969, two games utilized the new DPH rule for the very first time. Two newly formed expansion teams, the Montreal Expos and the Kansas City Royals, would participate in one such game, and the New York Yankees and Washington Senators in the other. On March 26, 1969, Major League Baseball nixed the idea for the time being. However, a four-year trial in which the International League and four other minor leagues started using the DH for their games began that year. 
Like other experimental baseball rule changes of the 1960s and 70s, the DH was embraced by Oakland A's owner Charlie O. Finley. On January 11, 1973, Finley and the other American League owners voted 8–4 to approve the designated hitter for a three-year trial run.  On April 6, 1973, Ron Blomberg of the New York Yankees became the first designated hitter in Major League Baseball history, facing Boston Red Sox right-handed pitcher Luis Tiant in his first plate appearance. "Boomer" Blomberg was walked.  The result of the first season of the DH was that the American League posted a higher batting average than the National League, something which has remained consistent to this day.
In response to increases in American League attendance because of the designated hitter[ citation needed], the National League held a yes/no vote on August 13, 1980, to determine whether or not the league would adopt the designated hitter. A majority of the 12 member teams was necessary to pass the rule, and the measure was expected to pass. However, when the teams were informed that the rule would not come into effect until the 1982 season, Philadelphia Phillies vice president Bill Giles was unsure of how the team owner, Ruly Carpenter, wanted him to vote. Unable to contact Carpenter, who was on a fishing trip, Giles was forced[ by whom?] to abstain from voting. Prior to the meeting, Harding Peterson, general manager for the Pittsburgh Pirates, was told to side with the Phillies.[ clarification needed] The final tally was four teams voting for the DH (Atlanta Braves, New York Mets, St. Louis Cardinals, and San Diego Padres), five votes against (Chicago Cubs, Cincinnati Reds, Los Angeles Dodgers, Montreal Expos, and San Francisco Giants), and three abstentions (Philadelphia Phillies, Pittsburgh Pirates, and Houston Astros). Five days after that meeting, the Cardinals fired their general manager, John Clairborne, who was the leading proponent for the adoption of the DH rule, and the National League has not held another vote on the issue. 
As time passed, the designated hitter rule has ended up offering American League managers multiple strategic options in setting their teams' lineups: they can rotate the DH role among part-time players (for example, using a left-handed batter against a right-handed pitcher and vice versa) or they can employ a full-time designated hitter against all pitchers. It also allows them to give a healthy everyday player a partial day off, or to give an injured player the opportunity to bat without exposing him to re-injury while playing in the field.
In recent years, full-time DHs have become less common, and the position has been used to give players a partial off-day, allowing them to bat but rest while the other team is batting. This option comes in very handy when a team's lineup includes several older players. The 2012 Yankees, for example, rotated five players through the DH spot during the season, all of them over 34 years old.  Only a handful of players compile over 400 at-bats as a DH each year.
With the Houston Astros having moved to the American League for the 2013 MLB season, which requires interleague play season-round (as well as the Astros to start using a DH full-time), there is debate within MLB to unify the rules of the two leagues, with either the American League returning to its pre-1973 rules and having the pitcher hit, like the National League, or the National League adopting the DH. 
In January 2016, MLB Commissioner Rob Manfred indicated that consideration was given to the National League adopting the DH for the 2017 season when a new collective bargaining agreement would take effect.  However, he later backtracked on this statement to say that he does not see unification coming any time soon.  Accordingly, the DH rule was not adopted by the National League for 2017.
Major League Baseball presents an annual award to the most outstanding designated hitter of the season, called the Edgar Martínez Award. Renamed for the former Seattle Mariners DH after his retirement in 2004, the Outstanding DH Award was introduced in 1973 and has been handed out every season since, except 1994 due to a players' strike. Notable winners include Martínez (five times) and David Ortiz (seven times, five consecutively).
DHs have generally not made much impact on the Major League Baseball Most Valuable Player Award or National Baseball Hall of Fame voting, because of the relative rareness of the full-time DH and the fact that the DH does not contribute on defense.
During the 1993 season Paul Molitor became the first player to win the World Series Most Valuable Player award while playing 137 of 160 games (85.63%) as a designated hitter; David Ortiz did the same in 2013. Ortiz was also the first designated hitter to win the ALCS MVP in 2004. Ortiz and Molitor played games at first base in their World Series MVP seasons, leaving Hideki Matsui in 2009 as the only World Series MVP to never play the field that season.
Among Hall of Famers, Paul Molitor and Jim Rice were, until 2014, the only inductees to even have played at least 25% of their games at DH. In 2014, Frank Thomas became the first Hall of Famer who played the majority of his games at DH. In 2018, Edgar Martínez was elected to the Hall of Fame having played over 70% of his games at DH.
There is considerable debate over whether or not the designated hitter rule should be removed,   while some  have argued that the National League should adopt it. On the other hand, there are also fans who enjoy the fact that the American and National Leagues use different rules. Two generations of fans of American League teams have grown up with the designated hitter rule being in place, so some may consider the designated hitter to be as much a traditional part of baseball as the pitcher taking his turn at bat is for fans of National League teams.
Critics often argue that use of the designated hitter introduces an asymmetry to the game by separating players into classes, creating offensive and defensive specialization more akin to American football. Opponents of the rule believe it effectively separates pitchers, other fielders, and designated hitters into separate roles that never cross, possibly causing issues with promoting ' batting cage' players whose scope of experience is extremely limited. However, when the pitcher bats alongside everyone else, all nine players must take turns at the plate and in the field, and the hybridization of roles requires that everyone knows other roles in addition to their own.
The designated hitter rule also changes managerial strategy in late innings. In the National League, a manager must decide when to let a pitcher bat or remove him, as well as with whom to pinch-hit and where or if that player should take the field afterward. When the decision to remove a pitcher is made, the manager may also elect to double switch, delaying the new pitcher's turn at bat. A designated hitter reduces the need for late-inning pinch hitters. 
Advocates of the designated hitter   point to the fact that it has extended many careers, and, in a few cases, created long, productive careers for players who are weak fielders or have a history of injuries, such as Edgar Martínez and David Ortiz. Hall of Fame members George Brett, Carl Yastrzemski, Paul Molitor, and Martinez continued their careers longer than they ordinarily would have without the rule.  Barry Bonds, who spent his entire career in the National League and actually won eight Gold Gloves earlier in his career, was used strictly as a DH later in his career when the San Francisco Giants played away interleague games due to his poor fielding.  Some believe that extending careers of older players is less of an advantage and more of a disadvantage, filling spots that otherwise may have been taken by younger players who end up not finding a place in the major leagues.
Interleague play has added a new wrinkle to the controversy. Some feel that it is not fair to ask an AL team to play without their DH when their roster has not been set up to do so, or on the other hand, to ask an NL team to use a DH when they may not have an appropriate player. Major League Baseball Commissioner Bud Selig once proposed that the road team's rules should be followed for interleague games in order to combat this advantage for the home team, but the idea has not received traction. 
The decline of pitcher Chien-Ming Wang due to an interleague game injury has been cited in support of the designated hitter. On June 15, 2008, Wang, at the time one of the Yankees' best pitchers, was taken out of an interleague game versus the Houston Astros due to a right foot injury he sustained while running the bases, something he was not used to doing, since pitchers do not bat in the American League. Wang was diagnosed with a torn Lisfranc ligament and a partial tear of the peroneus longus of the right foot. The cast was removed on July 29, but the extensive rehabilitation process prevented Wang from being an effective pitcher at the major league level since. Yankees part-owner Hank Steinbrenner showed frustration with pitchers having to bat in the National League and suggested that the League "join the modern age". 
Among Minor League Baseball teams, Rookie and Single-A level leagues use the DH in all games. At the Double-A and Triple-A level, when both teams are National League affiliates, pitchers may bat. In the Pacific Coast League, pitchers only hit when both clubs are NL affiliates and both clubs agree to have their pitchers hit. The reason for the difference is that as players get closer to reaching the majors, teams prefer to have the rules mimic, as closely as possible, those of the major league teams for which the players may soon be playing. 
The DH is used in most professional baseball leagues around the world. One notable exception is the Central League of Japan, where pitchers bat as they do in the National League. Japan's Pacific League adopted the designated hitter in 1975. When teams from different leagues play against each other in the Japan Series or interleague games, the DH rule is adopted if the Pacific League's team hosts the game. The DH rule is used in the Japanese minor leagues.
In American high schools and other amateur baseball leagues that use National Federation of State High School Associations (NFHS) rules, a DH may bat in place of one player in any position, not just a pitcher. Many coaches use a designated hitter in place of the weakest hitter in the lineup, if they use one at all. In amateur baseball, many pitchers are also good hitters and will often play another position (or even DH) when not pitching. Japanese high school baseball is one of the few amateur baseball leagues in the world that has never used the designated hitter rule at all. In high school baseball in South Korea, the rule has been adopted since 2004. American Legion rules, on the other hand, allow the DH only to bat for the pitcher; prior to 1995, the use of the DH was not allowed in Legion baseball at all.
In college baseball, NCAA rules state that the designated hitter must hit for the pitcher, but in many instances the pitcher is also a good hitter, and the coach may elect to let the pitcher bat in the lineup. If the pitcher opts to bat for himself, he is treated as two separate positions – a pitcher and a designated hitter (abbreviated P/DH on the lineup card) – and may be substituted for as such (i.e. if he is removed as the pitcher, he may remain as the designated hitter and vice versa). However, if a player who starts a game as a P/DH is relieved as the starting pitcher, he may not return to the mound even if he remains in the game as the DH, and he may not play any other defensive position after being relieved as the pitcher unless he immediately moves to another defensive position, in which case the new pitcher must assume the spot in the batting order of the fielder the P/DH substituted for, and the DH is lost for the remainder of the game. Conversely, a player who begins the game as the DH, but not as the pitcher, may come into the game as a reliever and remain as the DH (in effect becoming a P/DH), be relieved on the mound later in the game but continue to bat as the DH.
In Little League Baseball, the DH is not used. However, a league may adopt a rule which requires all players present and able to play to be listed in the batting order (such that the order contains more than nine players), and thus all players will have a turn to bat even when they are not assigned a fielding position. Players in the batting lineup without a position on the field are given the position designation extra hitter (EH), a position seen occasionally in other amateur organizations (both youth and adult).
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