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Baseball Rule Changes: Breaking Down Differences For Affiliated, Partner Leagues In 2021

The return of the minor leagues for 2021 has brought with it a slew of new rules for different levels, both among affiliated and partner leagues.

Here’s a rundown of all the rules changes that have been announced so far.



The size of bases will increase from 15-by-15 inches to 18-by-18 inches. By doing so, Major League Baseball expects the slightly shorter distance between bases will lead to a higher success rate on stolen base attempts and more infield hits on grounders and bunts. The larger bags may also decrease the number of collisions/spiking incidents at first base. The rule will be in place in Triple-A East for the first half of the season and in Triple-A West for the second half of the season.


Efforts will be made to limit shifting and the ways teams can position their infielders. All teams must have at least four players on the infield during play, which is defined as having both feet “completely in front of the outer boundary of the infield dirt.” If the results of the rule change are deemed positive in the first half, the league may require teams to have at least two players on either side of second base in the second half.


Teams will implement a rule which was in play in 2019 in the Atlantic League. In these leagues, pitchers will be required to disengage the rubber completely before throwing to any base. With this rule in play, the Atlantic League saw a significant uptick in stolen bases. The rule was scheduled to be implemented in the minor leagues in 2020 before the coronavirus pandemic canceled the season.


Pitchers will be limited to two step-offs or pickoff attempts per plate appearance. If a pitcher tries a third pickoff in a plate appearance, the move will be considered a balk unless the runner is successfully picked off. Depending on the results of the rule change, MLB will consider reducing the number of pickoffs or step-offs per plate appearance to just one.

At Low-A Southeast only, select games will use the automatic ball-strike system, which was tested in both the Atlantic League and Arizona Fall League in 2019. The strike zone will be a two-dimensional box at the front of the plate. The size of an individual batter’s strike zone will be determined based on the batter’s height.

At Low-A West only, teams will add timers to enforce time between pitches with a 15-second clock.

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Atlantic League

The Atlantic League will trial two new rules at the request of MLB.

1) It will use a “double hook” designated hitter. Once the starting pitcher is pulled from the game, the designated hitter is also pulled, meaning that relief pitchers will have to bat for themselves (or be pinch hit for).

2) In the second half of the season, the mound will be moved back one foot to 61 feet, 6 inches from home plate.

In addition, the league will continue to use some other rules innovations.

The Atlantic League will once again use an Automated Ball-Strike System (robo-umps) using the newest generation of Trackman tracking software to call balls and strikes. The home plate umpire will relay the calls after hearing them through an earpiece.

The league will once again use 18-inch bases at first, second and third. Those bases (used in 2019) are three inches larger than traditional bases.

Atlantic League infielders must have both feet on the dirt of the infield when a pitch is released.

The league will hold between innings breaks to 1 minute, 45 seconds.

The extra-inning tiebreaker will continue, with a runner being placed on second to begin every extra inning.

Each team will have three timeouts per every nine-inning game, as well as an additional timeout in the 10th and one additional timeout every three innings after the 10th.

Atlantic League pitchers will have 15 seconds to throw a pitch, using a pitch clock.

Pioneer League

Instead of playing extra-innings, all games tied at the end of the regulation nine innings will be settled by a home run derby. Each team will select one player who gets five pitches (thrown by a batting practice pitcher). The team whose player hits more home runs from those five pitches wins the game. If the two teams are still tied after those five pitches, there is a sudden death home run derby where they go back and forth one swing at a time.

Teams can use a designated pinch hitter once per game, where a player who has not played in the game can pinch hit for someone in the lineup. The player who was pinch hit for can return to his defensive position in the next half inning and remain in the game. The pinch hitter is ineligible to return to the game if the initial player he pinch hit for remains in the game. This can only be used once per game,

Teams can also use a designated pinch runner once per game. Similar to the designated pinch hitter, the pinch runner can substitute for a runner on base and the initial player can return to his defensive position at the end of the half inning. The pinch runner is then ineligible to return to the game again if the initial player remains in the game. This can only be used once per game.

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