Alloy bats are generally constructed with a one-piece design out of aluminum or aluminum that is mixed with other metals to make a stronger product. The advantage to this strength is that it allows alloy bats can have thinner, more responsive barrel walls. Composite bats, on the other hand, are made out of a mixture of carbon fiber, graphite, fiberglass, and sometimes Kevlar. Many leagues are beginning to ban composite-barreled bats because, as they are used, the carbon fiber weave begins to become more responsive and much "hotter" than the current standards allow. Check your league rules before buying a composite-barreled bat. Hybrid bats feature a two-piece design in which an alloy barrel is bonded to a composite handle. This makes the handle lighter and allows the alloy barrel to be made longer than on a traditional alloy bat.