Major League Baseball rule
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.
Interleague play and exhibitions
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 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.