Greg A. Harris was one of few major league pitchers in the modern era to pitch with both his left and his right arm, though he only did so in a single Major League game. A natural right-hander, by 1986 he could throw well enough left-handed that he felt capable of pitching with either arm in a game. Harris did not throw left-handed in a regular-season game until September 28, 1995, the penultimate game of his career. Pitching for the Montreal Expos against the Cincinnati Reds in the ninth inning, Harris retired Reggie Sanders pitching right-handed, then switched to his left hand for the next two hitters, Hal Morris and Ed Taubensee, who were both left-handed batters. Harris walked Morris but got Taubensee to ground out. He then went back to his right hand to retire Bret Boone to end the inning.
Pat Venditte regularly pitches with both arms. Venditte was drafted by the New York Yankees, played for the Seattle Mariners, and now plays for the Philadelphia Phillies. While with the Staten Island Yankees, the Yankees' Single-A affiliate, when he opposed switch hitter Ralph Henriquez, Venditte switched his modified glove to his left arm. (Hitters traditionally derive advantages from batting from the opposite side of the plate to the pitcher's throwing arm.) Henriquez then switched to batting left-handed, and a series of changes continued for several minutes. This prompted the PBUC (Professional Baseball Umpires Corporation) to issue a new rule about switch-pitching. In short, switch-pitchers must indicate to the umpire, batter, and any runners the hand with which they will use to pitch. The pitcher must continue using this hand for the duration of the at bat, with some exceptions for injury and the use of pinch hitters. Following this choice, batters can then select with which hand they will bat. 
In the collegiate ranks, Matt Brunnig (Harvard class of 2006-07) was able to pitch over 85 mph left-handed and over 90 mph right-handed, but only pitched with both arms in the same game a few times. In college, he pitched more from the right side as a starter and pitched some relief as a lefty although he did start one game left-handed. When playing the outfield after a start he would typically play the position with the other arm to rest the arm he just pitched with.
Switch-throwers are commonly taught to switch-throw at a young age. For instance, Venditte's father trained him in ambidextrous throwing from the age of three and Brunnig's father taught him from age five.
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