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From BR Bullpen
A pitcher's repertoire is the range of pitches that he throws, and is one of his defining characteristics. Repertoire is not just a list of a pitcher's pitches (e.g. he throws a fastball, curveball, and change-up) but also includes information about their quality and the way he uses them (e.g. he rarely throws his mediocre fastball and depends instead on a slow 12-6 curveball and an effective change-up). A pitcher may be able to throw a specific pitch but use it so rarely that it isn't considered to be part of his repertoire. For instance, John Smoltz is able to throw a knuckleball and has occasionally used it in a game, but it isn't part of his normal repertoire.
As a rule of thumb, starting pitchers tend to have a larger repertoire than relief pitchers. Starters are expected to face the same batters three or four times over the course of a game. Having a large repertoire allows a starter to pitch to batters differently each time they face. In contrast, relief pitchers are not expected to pitch even once through the batting order, so they are encouraged to use only their very best pitches.
Although a pitcher's repertoire is sometimes treated as a fixed characteristic, most pitchers will change their repertoires over the course of their careers. It's very common, for instance, for pitchers' fastballs to lose some speed and effectiveness as they age, and successful pitchers compensate by developing new pitches. The variability of pitchers' repertoires is one reason that their career paths are unpredictable.