Section 1.10 of the Official Rules of Major League Baseball (MLB) places strict limits on the design, diameter and length of wooden bats for competitive play. The rule book does not impose a limit on the bat's weight, however.
MLB rules state that the bat “shall be a smooth, round stick not more than 2 3/4 inches in diameter at the thickest part and not more than 42 inches in length.” Each bat incorporates one solid piece of wood. The rules bar laminated or “experimental bats” from professional games—both regular season and exhibition—until the MLB Rules Committee approves the manufacturer's design.
Rules permit an indentation in the end of a bat's barrel that can measure up to 1 inch in depth, no wider than 2 inches and not less than 1 inch in diameter. The indentation must be curved, and the indentation can contain no foreign substance.
A hitter may cover or treat his bat's handle—to a height not more than 18 inches from its bottom end—with any material or substance he wishes to improve his grip. The bat is removed from play if any material (such as pine tar) extends beyond the 18-inch limitation. If the umpire discovers that the bat violates this rule after a player has used it in a game, he shall declare the hitter out and/or eject him from the game.
Colored bats are not permitted for play in a professional game unless they receive prior approval from the Major League Baseball Rules Committee.