the permanent substitution rule; if the defense has more than nine players on the field at any time, the umpire must determine who is the tenth player, and that player is ejected from the game. Many amateur leagues allow a starting player who was removed to return to the game in the same position in the batting order under a re-entry rule. Youth leagues often allow free and open substitution to encourage player participation. Pitchers as batters, and the designated hitter Pitching is a specialized skill, particularly in the collegiate and professional ranks; so most pitchers are relatively poor hitters, or, those who were skilled batsmen are simply unable to adequately hone their hitting skills to be comparable to everyday position players. As a result, unless keeping a given pitcher in the game is a higher priority than the prospect of immediate offense, it is common to substitute for a pitcher when he is due to bat. This pinch hitter is typically then replaced by a relief pitcher when the team returns to the field on defense. A more complicated tactic is the double switch, in which a pitching change is accompanied by the simultaneous replacement of another fielder. If the pitcher is due to bat soon, and the outgoing fielder batted recently, the new pitcher will take the outgoing fielder's place in the batting order, thus delaying his next time at bat. A common variation on this involves the introduction of a player who has just pinch hit (or pinch run for the pinch hitter) into the defensive alignment; unless this player becomes his team's next pitcher, another field player departs the game, and the new pitcher then assumes that player's place in the batting order. Many leagues allow designated hitters, notably Major League Baseball's American League (which instituted the Designated Hitter in 1973 to boost offensive output). A designated hitter (or DH) is a player whose sole purpose is to hit when it would normally be the pitcher's turn (or, in some leagues, if the pitcher is a good batter, another weaker batter). This is not considered a substitution, but rather a position, albeit a purely offensive one. A designated hitter does not play in the field on defense and may remain in the game regardless of changes in pitchers. If the designated hitter is moved to a fielding position, the team loses the DH, and the fielder