Beginning with the 2009 season, baseball bats with a diameter of 2-5/8 (two-and-five-eighths) inches are now the standard for all teenage baseball divisions (Junior, Senior and Big League).  Baseball bats with a diameter of 2-3/4 (two-and-three-fourths) inches are no longer permitted for use in the Junior League Division of Baseball. Previously, these bats could be used in Juniors, but not in Senior or Big League. 
 
Background
 
A committee at USA Baseball is in the process of developing a standard for bats used in the Junior League Division. Currently, there is no standard in place as there is for bats in Little League Baseball (Majors) and below, and for bats used in Senior League and Big League.  (USA Baseball is the governing body for amateur baseball in the U.S. Little League Baseball is a member of USA Baseball.)  This step taken by Little League, and to be taken by other youth baseball organizations in the coming year or two, is a first step in that process and a way to prepare for the new standard. Eventually, for the Junior League age group, only bats with a diameter of 2-5/8 (two-and-five-eighths) inches will be manufactured to meet the new standard when it is ready for implementation.  (Note: The standard for bats in the Little League Baseball [Majors] Division and below is the Bat Performance Factor [BPF]. This is a measure of a non-wood bat's performance relative to wooden bats.)
 
In the Senior and Big League Divisions, the standard is the Ball Exit Speed Ratio (BESR), with its "minus-3" requirement. The overall weight (in ounces) of a bat can be no more than three ounces below the overall length (in inches) of the bat. This mandate remains in effect in Senior and Big League Baseball divisions, but is not a requirement in Junior Baseball.

In effect for the 2009 season is the requirement that all non-wood bats used in Little League Baseball (Majors) Divisions and below shall be printed with a BPF of 1.15 or less.  This stipulation has been included in the Official Rules and Regulations for the past few years, leading up to 2009, so that leagues, parents, and bat manufacturers could prepare for it.  Most non-wood bats manufactured over the past several years for the age groups involved in Little League Baseball (Majors) and below have been permanently imprinted with the BPF. In fact, very few bats sold today with a diameter of 2-1/4 (two and one-quarter) inches are not printed with the BPF rating.  So, starting with the 2009 season, bats printed with a BPF higher than 1.15, or without a BPF printed (or not legible) on the bat, are not permitted in these divisions.