Baseball is governed by a specific set of rules that controls the basic procedures and overall flow of the game. The first set of rules contained a basic list of procedures, but new rules have been implemented over time as the game has evolved. Each league or organization enforces its own set of rules, but rules regarding carrying a bat are common among all leagues. While the Major League Baseball (MLB) rulebook doesn't directly define carrying a bat, it does cover offensive interference as a result of carrying the bat and defines the use of an illegal bat.
Batter Interference
Major League Baseball doesn't have a specific rule regarding carrying a bat after making contact with the ball. However, if a player is carrying a bat and it results in interfering with the defense, the batter will be declared out. Rule 6.06 (c) of the 2010 MLB rulebook defines batter interference as any act that "interferes with the catcher's fielding or throwing." If batter interference occurs, the umpire will call interference, and the batter will be declared out. When this occurs the ball is dead and all runners must return to the last legally touched base at the time of the interference call.
Illegal Bat
Most leagues specifically restrict the use of illegal bats. Rule 6.06 (d) of the MLB rule book defines an illegal bat as a bat which, in the umpire's judgment, has been altered or tampered with in a way that results in improved performance. A player may not carry an illegal bat to the batter's box, and can be called out for using or attempting to use an illegal bat. If the player makes contact with the ball and the umpire subsequently determines the bat to be illegal, the batter is declared out and ejected from the game, and all other base runners may not advance.
Offensive Interference
After making contact with a pitch, the batter may not interfere with any defensive players while running the bases. According to Rule 2.00 (a) of the MLB rulebook, if the runner is carrying the bat and interferes with a defensive player, the umpire will call interference and the offending player will be declared out. Additionally, if the batter hasn't reached first base at the time of the interference call, all other runners must return to the base occupied during the pitch. Runners are returned to the last legally touched base if the player is called for interference after reaching first base.
League Rules
Some leagues may impose special rules regarding carrying illegal bats or carrying the bat while running the bases. For example, the 2011 American Legion baseball rules require all bats to have an approved certification mark signifying "BESR" or "BBCOR." Along with these marks, the bats are allowed a maximum length of 36 inches and a maximum barrel diameter of 2 5/8 inches. If a bat is considered illegal, the umpire will remove the bat from the game and the batter will be declared out.



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