Enforcement
While baseball rules provide a precise definition for the strike zone, in practice it is up to the judgment of the umpire to decide whether the pitch passed through the zone. Historically, umpires often call pitches according to a contemporary understanding of the strike zone rather than the official rulebook definition.
Many factors have contributed to the divergence of the official and conventional strike zones in Major League Baseball. Changes began in the 1970s, when umpires upgraded their chest protection in favor of more compact vests allowing them more movement. Crouching lower meant lowering their line of vision, and caused the boundaries of the strike zone to sink lower. Thus, the strike zone was often enforced such that pitches above the waist were balls, and pitches a few inches outside of home plate were called strikes. As pitchers lost the higher strike zone, they began throwing lower and to the outside, which caused hitters to move much closer to the plate.
At the same time, there was a shift in attitude among both players and league officials regarding pitches thrown inside. While pitchers of the 1960s such as Bob Gibson regarded it a pitcher's right to throw high and inside, later batters were more likely to take offense at such treatment. Major League Baseball also tightened its rules prohibiting pitchers from intentionally hitting batters, removing the warning pitchers formerly received before being ejected from a game. Soon, hitters moved closer to the plate and looked for the ball outside.
In 2001, Major League Baseball directed its umpires to call pitches according to the official definition rather than the conventional one. Umpires were to call "high" strikes and "inside" strikes, while pitches just off the outside part of the plate were to be called balls. The umpires demonstrated limited compliance for a time, but before long the de facto strike zone had returned to the conventional definition. Shortly thereafter, Major League Baseball began privately evaluating umpires based on the QuesTec pitch-tracking system. Most umpires, players and analysts, including the authors of a University of Nebraska study on the subject, believe that due to QuesTec, the enforced strike zone in 2002-2006 was larger compared to the zone in 1996-2000 and thus closer to the rulebook definition. Some commentators, such as Tim Roberts of covers.com, believe that the zone has changed so much that some pitchers, such as Tom Glavine, have had to radically adjust their approach to pitching for strikes. In 2003, a frustrated Curt Schilling took a baseball bat to a QuesTec camera and destroyed it after a loss, saying the umpires shouldn't be changing the strike zone to match the machines.
In 2009, a new system called Zone Evaluation was implemented in all 30 Major League ballparks, replacing the QuesTec system; the new system records the ball’s position in flight more than 20 times before it reaches home plate. Much of the early resistance from Major League umpires to QuesTec had diminished and the implementation of the new Zone Evaluation system in all the parks went largely unnoticed. Like the old system, the new system will be used to grade umpires on accuracy and used to determine which umpires receive post season assignments.
"You can't pitch fastballs inside anymore, and you never get a called strike with a fastball inside," said former pitcher Gene Garber.
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