Rookie and single-A level leagues in Minor League Baseball employ the DH rule in all games. The designated hitter rule is not accessible at the Double-A and Triple-A levels when both clubs are National League affiliates, but it is in force elsewhere.
The decision to implement the DH rule was made by the minors' parent organization, the American League (AL) teams, in order to make play easier for outfielders who were being sacrificed to fill in for injured or tired players. The first season of its use was 1969. Before that time, if a player was removed from an offensive position (e.g., third base), the pitcher would then have a chance to hit instead. By making him the batter rather than the pitcher, managers reasoned, they could maintain a regular lineup card.
In addition, the DH allows each team one opportunity during each game to have its batter stand in the on-deck circle while the opposing manager makes a replacement player available. This gives each team an opportunity to get a new man in the batting order every time out without having to substitute pitchers. It also ensures that a pinch-hitter will be able to come into the game without having played defense earlier in the game. Pinch-hitters who come from the on-deck circle are said to be "activated."
The designated hitter (DH) rule is used in the same way as it is in the World Series and All-Star Game. In an American League stadium, both clubs utilize a designated hitter (DH) to bat for the pitcher. Pitchers from both clubs must bat at a National League venue. A batter can be replaced by a substitute player if necessary.
During World War II, the use of DHs became popular among American League (AL) teams as a way to save on playing time for regular batters. Before the war, most AL games lasted at least five hours from first pitch to last out because there were only eight games per team per season. During that period, several all-star games were played with both NL and AL players participating. The need for replacements due to military service was great, so AL owners decided to allow one regular player to be replaced by a substitute while still keeping his salary from their roster.
After the war, the use of DHs continued to grow among AL teams as a way to increase their scoring chances. By 1948, all AL games included at least one hitter batting for pitcher. That same year, two pitchers each made an appearance in the All-Star Game. From 1949-1951, three pitchers took turns hitting for each other. From 1952-present, one pitcher has been allowed to hit during any inning in which he is pitching.
The DH Regulation (rule 6.10 of the Major League Baseball Rules) has a few quirks: The DH is not required. In a game when a designated hitter would ordinarily be employed, a club may opt to bat their pitcher instead. A team can use only one pitcher and one hitter per inning, so if they choose to use the pitcher to hit, someone else will have to go to the plate.
However, since 2007, when it was first used by the Chicago Cubs during Game 1 of the National League Division Series against the Arizona Diamondbacks, no pitcher has been used to hit in an official MLB game.
The rule was introduced in April 1995 by the American League in response to concerns about the safety of batting pitchers. Before this rule change, several major league players had been injured while batting other players who were not intended to be targets but instead threw or batted balls aimed at their heads. For example, in 1989, the Detroit Tigers' Mark Fidrych was nearly killed when he was hit in the head by a pitch from Mike Bacsik of the Toronto Blue Jays. After these incidents, the AL decided that something needed to be done to protect its pitchers, so it created a new rule category - Designated Hitter.