Wood bats: types, when they're required, and sizing

Do wood bats need a stamp?

One-piece solid wood: almost never. It's legal without a mark in Little League, USABat leagues and high school. Multi-piece, laminated, bamboo and composite-wood bats usually do need a certification mark. College rules require every bat that isn't one-piece solid wood to be certified.

Leagues that require wood

• AABC 15U and up (Griffey, Mantle, Mattingly, Connie Mack, Stan Musial). Only the Baum Bat and Zinger composite are allowed.
• PONY Thorobred (23U)
• Many Perfect Game showcases (check the event)
• Summer college leagues like the Northwoods League
• MSBL/MABL and NABA national tournaments (composite wood and metal/wood allowed)

Maple, ash, birch or bamboo

• Maple: hard, dense, stiff. Durable, but can shatter.
• Ash: softer and more flexible, so it's more likely to break or dent and tends to splinter.
• Birch: in between, more flexible than maple and harder than ash.
• Bamboo: laminated strips, durable. Needs a mark in youth play.

Source: Rawlings: different wood baseball bats, USABat FAQ.