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In baseball, a left-handed specialist (also known as lefty specialist) is a relief pitcher who throws left-handed and specializes in pitching to left-handed batters, weak right-handed batters, and switch-hitters who bat poorly right-handed. Because baseball practices permanent substitution, these pitchers frequently pitch to a very small number of batters in any given game (often only one), and rarely pitch to strictly right-handed batters. Most Major League Baseball (MLB) teams have several left-handed pitchers on their rosters, at least one of whom is a left-handed specialist. A left-handed specialist is sometimes called a LOOGY (or Lefty One-Out GuY), a term which can be used pejoratively (coined by John Sickels).
The pitcher generally has an advantage when his handedness is the same as the batter's, and the batter has an advantage when they are opposite. This is because a right-handed pitcher's curveball breaks to the left, from his own point of view, which causes it to cross the plate with its lateral movement away from a right-handed batter but towards a left-handed batter (and vice-versa for a left-handed pitcher), and because batters generally find it easier to hit a ball that is over the plate. Furthermore, since most pitchers are right-handed, left-handed batters generally have less experience with left-handed pitchers. A left-handed pitcher may also be brought in to face a switch-hitter who generally bats left-handed, forcing the batter to shift to his less-effective right-handed stance or to take the disadvantages of batting left-handed against a left-handed pitcher. Research from 2011-2013 has shown that a pinch hitter (usually right-handed) is often used when a left-handed reliever is inserted in the game, thereby reducing or negating the pitcher's platoon split advantage. Only a handful of left-handed relievers face a higher percentage of left-handed batters than right-handed batters over the course of a season.
In 1991 in MLB, there were 28 left-handed relievers who were not their team's closer and pitched 45 or more games. Only four averaged fewer than an inning per appearance. From 2001 to 2004, over 75 percent of left handed relievers meeting those criteria averaged less than one inning. Left-handed reliever John Candelaria was one of the early specialists in 1991, pitching 59 games and averaged .571 innings. In 1992, he allowed no earned runs—excluding inherited runners—in 43 of the 50 games. Jesse Orosco became a left-handed specialist late in his 24-season career and retired at age 46. From 1991 to 2003, he never averaged more than an inning pitched per appearance.
During the 2013 MLB season, there were seven relief pitchers who averaged less than two outs recorded per appearance, all of whom were left-handed. Joe Thatcher, a left-handed specialist, appeared in 72 games with 39.2 innings pitched, and had the fewest outs recorded per appearance with 1.6.
The right-handed specialist (sometimes called a ROOGY, for Righty One-Out GuY) is less common than the left-handed specialist, but are occasionally featured. Naturally, these pitchers are themselves right-handed, and they almost always throw either sidearm or submarine. Examples include Chad Bradford, Joe Smith, Brad Ziegler  and Cody Eppley.