A bat is a stick used by the batter to hit the baseball. Baseball bats were traditionally made from ash, though other hardwoods have been used. Hollow metal (such as aluminum) and composite bats are now standard in most amateur baseball and some professional leagues outside of North America. The non-wooden bats can hit the ball much harder and further than wooden bats, and many leagues are now placing restrictions on their performance.
According to the Official Rules of Major League Baseball :
- (a) 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. The bat shall be one piece of solid wood. NOTE: No laminated or experimental bats shall be used in a professional game (either championship season or exhibition games) until the manufacturer has secured approval from the Rules Committee of his design and methods of manufacture. (b) Cupped Bats. An indentation in the end of the bat up to one inch in depth is permitted and may be no wider than two inches and no less than one inch in diameter. The indentation must be curved with no foreign substance added. (c) The bat handle, for not more than 18 inches from its end, may be covered or treated with any material or substance to improve the grip. Any such material or substance, which extends past the 18 inch limitation, shall cause the bat to be removed from the game. NOTE: If the umpire discovers that the bat does not conform to (c) above until a time during or after which the bat has been used in play, it shall not be grounds for declaring the batter out, or ejected from the game. (d) No colored bat may be used in a professional game unless approved by the Rules Committee.
Although made of a single piece of wood, bats are described as having several parts. At the end furthest from the hands the bat may have a cup, a circular indentation intended to make the bat lighter without loss of striking surface. The barrel or striking surface, of the bat is generally made as big around, or nearly as big around, as the rules will allow, and extends at that diameter for about 1/3 of the length of the bat. The area of the barrel that is the ideal point for hitting the ball is known as the sweet spot. The side of the bat that will be facing upward while the bat moves through the strike zone is branded with the maker's logo at about the position of the sweet spot. The bat should always be swung so that the grain is parallel to the bat's motion, as the bat is strongest in that direction; the brand helps the batter to orient the bat correctly. The barrel of the bat tapers down to a narrow handle which the batter grips. The handle ends in a small swelling called the knob that helps to prevent the bat from slipping out of the batter's hand when he swings. Most batters grip the bat so that the knob touches their bottom hand or even wrap their bottom hand around the knob. A few batters will "choke up", gripping the bat some distance above the knob for better control.
Bats are usually covered with laquer to protect the wood. The laquer may be clear or colored. The handle of the bat is often wrapped in string or tape to make it easier to grip. Some players prefer to coat the handle with sticky pine tar. The use of pine tar was highlighted in the infamous Pine Tar Game, when Yankees manager Billy Martin tried to have a critical home run by George Brett nullified because he had more than the legal amount of pine tar on his bat.
The very earliest bats seem to have been modeled on, or possibly even been, tool handles. They were made of the same kinds of wood, hickory and ash, and they were more or less cylindrical without the rapid taper from barrel to handle of modern bats. Even into the early professional game, bats were often made one-off by local woodworkers rather than in any systematic way. The bats were durable enough that they rarely broke and players wouldn't necessarily see the need to keep a spare available.
By about 1890 bats had begun to generally resemble their modern counterparts. Players recognized the advantages of a large barrel with a sharper taper to the handle, and the rules allowed modifying the handle to improve grip. A short-term experiment allowing the use of bats that were planed flat on one side was rejected. The rules on the construction of bats have scarcely changed since 1895, when the maximum legal diameter was increased from 2 1/2 to 2 3/4 inches.
Although the rules regarding bats have scarcely changed since 1895, practical construction of bats has changed considerably. Players in the late 19th and early 20th Century commonly used bats that weighed 3 pounds or more on the theory that a heavy bat could hit the ball further than a lighter bat. Other players used much lighter bats, often made of relatively soft wood. A few players, most notably Heinie Groh, used "bottle bats" that narrowed abruptly from the barrel to the handle rather than tapering gradually.
Players have gradually adopted lighter bats on the theory that bat speed rather than weight is the key to hitting the ball hard. Hickory wood, once a favorite bat material because of its density and strength, has been completely abandoned as a material for bats. Most batters today use bats that are from 33-36 inches long and have a drop of -3, meaning that their weight in ounces is about three less than their length in inches. The bats used by Mark McGwire and Barry Bonds to break the single season home run record weigh almost a third less than those used by Babe Ruth when he set the same record.
Another very recent trend in bat construction is the move from ash to maple wood. Maple is supposed to be more durable than ash, and players believe that it is harder and thus able to hit the ball further. The fact that Barry Bonds was among the first players to use a maple bat contributed to their popularity in the early 2000s. The greater hardness and durability comes at a cost in weight, so maple bats are often made slightly smaller in diameter than ash bats to keep their weight down. Maple bats have become controversial because of their tendancy to shatter in sharp shards when they break, posing an injury risk to spectators, players and umpires. There were calls to ban them in 2008, after a series of incidents in the early season. Yellow birch is another type of wood that has recently gained popularity among players; it is more flexible than maple, but harder than ash.
- Thomas Boswell: "Magic Wands and Louisville", in How Life Imitates the World Series, Penguin Books, New York, 1982, pp. 165-169.
- Ray Glier: "Louisville Slugger losing grip as bat of choice", USA Today Sports, October 1, 2013 
- David Magee and Philip A. Shirley: Sweet Spot: Years of Baseball and the Louisville Slugger, Triumph Books, Chicago, IL, 2009.
- Ben Walker: "Properties of Baseball Bats", in The Baseball Research Journal, SABR, Volume 39, Number 1 (Summer 2010), pp. 113-121.