A knuckleball (or knuckler for short) is a baseball pitch with an erratic, unpredictable motion. The pitch is thrown so as to minimize the spin of the ball in flight. This causes vortices over the stitched seams of the baseball during its trajectory, which in turn can cause the pitch to change direction—and even corkscrew—in mid-flight. This makes the pitch difficult for batters to hit, but also difficult for pitchers to control. The challenge also extends to the catcher, who must at least attempt to catch the pitch, and the umpire, who must determine whether the pitch was a strike or ball.
The identity of the first pitcher to throw a knuckleball is uncertain, but it appears to have been developed in the early 20th century. Lew "Hicks" Moren  of the Philadelphia Phillies was credited as its inventor. However, Eddie Cicotte apparently also came up with the pitch while at Indianapolis, and brought it to the major leagues two years later in 1908. Since Cicotte had a much more successful career (and also gained later notoriety as one of the players implicated in the Black Sox scandal), his name is the one most often associated with the invention of the pitch today.
As used by Cicotte, the knuckleball was originally thrown by holding the ball with the knuckles, hence the name of the pitch. Ed Summers, a Pittsburgh teammate of Cicotte who adopted the pitch and helped develop it, modified this by holding the ball with his fingertips and using the thumb for balance. This grip can also include digging the fingernails into the surface of the ball. The fingertip grip is actually more commonly used today by pitchers who throw the knuckleball, like Boston's Tim Wakefield, who has a knuckleball with a lot of movement, or Hall of Famer Phil Niekro, who had a very effective knuckler and knucklecurve. However, youngsters with smaller hands tend to throw the knuckleball with their knuckles. Sometimes these youngsters will even throw the knuckleball with their knuckles flat against the ball, giving it less spin but also making it difficult to throw any significant distance.
Regardless how the pitch is gripped, the purpose of the knuckleball is to avoid the rotational spin normally created by the act of throwing a ball. In the absence of this rotation, the ball's trajectory is significantly affected by variations in airflow caused by differences between the smooth surface of the ball and the stitching of its seams. The asymmetric drag that results will tend to deflect the trajectory toward the side with the stitches.
Over the distance from the pitcher's mound to home plate, the effect of these forces is that the knuckleball can "flutter," "dance," "jiggle," or actually curve in two opposite directions over its flight. A pitch thrown completely without spin is actually less desirable, however, than one with only a very slight spin (so that the ball completes perhaps between one-half and one rotations on its way from the pitcher to the batter). This will cause the position of the stitches to change somewhat as the ball travels, and therefore the drag that gives the ball its motion, thus making its flight even more erratic. Even a ball thrown without rotation will "flutter" somewhat, due to the 'apparent wind' it feels as its trajectory changes throughout its flight path.
The unpredictability of a knuckleball is such that it can be very easy to hit hard if it doesn't change direction in mid-flight. Since it normally only travels to , far slower than the average major league fastball, it can be hit very hard if there is no movement. To reduce the chances of having the knuckleball get hit for a home run, some pitchers will impart a slight topspin so that if no force causes the ball to dance it will move downward in flight. Another drawback is that runners on base can usually advance more easily than if a conventional pitcher is on the mound. This is due to both the knuckleball's low average speed and erratic movement, which forces the catcher to keep focusing on the ball even after the runner takes off. A few knuckleball pitchers, such as Hoyt Wilhelm and Tim Wakefield, required their own catchers to catch knuckleballs.
Since it developed during a period when the spitball was legal and commonly used, and was similarly surprising in its motion, the knuckleball was sometimes called the "dry spitter". Cicotte was widely reported to throw both the knuckleball and a variant on the spitball known as a "shine ball" (because he would "shine" one side of a dirty ball by rubbing it on his uniform). However, Cicotte called the shine ball "a pure freak of the imagination", claiming that he did this to disconcert hitters but that the pitch was still a knuckleball.
Other names for the knuckleball have generally alluded to its motion and slower speed. These include the flutterball, the floater, the dancer, the butterfly ball, the ghostball, and the bug.
The knuckle curve has a somewhat similar name because of the grip used to throw it (also with the knuckles or fingernails), but it is generally thrown harder and with spin. The resulting motion of the pitch more closely resembles a curveball, which explains the combination name. Toad Ramsey, a pitcher from 1885–1890, is credited in some later sources with being the first knuckleballer, apparently based primarily on accounts of how he gripped the ball; however, based on more contemporary descriptions of his pitch as an "immense drop ball", it may be that his pitch was a form of knuckle curve. Two later pitchers, Jesse Haines and Freddie Fitzsimmons, were sometimes characterized as knuckleball pitchers even by their contemporaries, but in their cases this again refers to a harder-thrown, curving pitch that would probably not be called a knuckleball today. Retired pitcher Mike Mussina threw a knuckle curve.
When originally developed, the knuckleball was used by a number of pitchers as simply one pitch in their repertoire, usually as part of changing speeds from their fastball. It is almost never used in a mixed repertoire today, however, and some believe that to throw the knuckleball effectively with some semblance of control over the pitch, one must throw it more or less exclusively. At the same time, pitchers rarely focus on the knuckleball if they have reasonable skill with more standard pitches, so knuckleball pitchers have become quite rare.
However, the knuckleball does provide some advantages to its practitioners. It does not need to be thrown hard (in fact, throwing too hard may diminish its effectiveness), and is therefore less taxing on the arm. This means knuckleball pitchers can throw more innings than orthodox pitchers, and are able to pitch more frequently because they require less time to recover after having pitched. The lower physical strain also gives them the potential for greater career longevity, as some have continued to pitch professionally well into their forties, such as Tim Wakefield, Tom Candiotti, and the Niekro brothers. In addition, some pitchers (such as Jim Bouton) have had success as knuckleballers after their ability to throw hard declined.
Hoyt Wilhelm, Phil Niekro and Jesse Haines, three pitchers who primarily relied on the knuckleball, have been inducted into the Baseball Hall of Fame. Niekro was given the nickname "Knucksie" during his career. Other prominent knuckleball pitchers have included Joe Niekro (Phil's brother), Charlie Hough, Dave Jolly, Ben Flowers, Wilbur Wood, Tom Candiotti, Bob Purkey, Steve Sparks, Eddie Rommel and Tim Wakefield. During the 1945 season, with talent depleted by call-ups to fight in World War II, the Washington Senators had a pitching rotation which included four knuckleball pitchers (Dutch Leonard, Johnny Niggeling, Mickey "Itsy Bitsy" Haefner and Roger Wolff) who combined for 60 complete games and 60 wins, carrying the Senators to second place.
, Wakefield of the Boston Red Sox and R.A. Dickey of the New York Mets are the only knucklers in the big leagues, though minor leaguers Charlie Zink of the Rochester Red Wings and Charlie Haeger of the Albuquerque Isotopes also throw the knuckleball. In November 2008 it was announced that 16 year old knuckleballer Eri Yoshida was drafted as the first woman ever to play in Japanese professional baseball for the Kobe 9 Cruise of the Kansai Independent Baseball League. On March 2, 2010, she trained with Tim Wakefield at the Boston Red Sox minor league training facility. and on April 8, 2010, she signed with the Chico Outlaws, debuting on May 29, 2010. Detroit Tigers reliever Eddie Bonine also throws a knuckleball, though he does so infrequently as compared to pitchers who use it as a primary pitch. Lance Niekro, son of Joe Niekro, attempted to convert from a position player to a knuckleball pitcher. He started the 2009 season with the Gulf Coast League Braves but is currently listed as a free agent.
The unpredictable motion of the knuckleball makes it one of the most difficult pitches for a catcher to handle. Catchers tend to be charged with a significantly higher number of passed balls when a knuckleball pitcher is on the mound. A team will sometimes employ a catcher solely for games started by a knuckleballer. The "knuckleball catcher" is equipped with an oversized knuckleball catcher's mitt, similar to a first baseman's glove; Doug Mirabelli, formerly of the Red Sox, actually used a softball catcher's mitt. The Boston Red Sox did this fairly systematically in their 2004 world championship season, with Doug Mirabelli regularly catching in place of Jason Varitek when Tim Wakefield was pitching. Jason Varitek would sometimes catch for Wakefield although it proved to be disastrous. In 2005, Wakefield's ERA was over 9.00 when Varitek caught for him. The Wakefield tradition of a 'Specialist Catcher' continued into the 2008 season following the signing of Kevin Cash and 2009 saw George Kottaras fulfill this role. On August 26, the first time Victor Martinez caught Wakefield, he used a first baseman's glove, instead of a regular catcher's mitt.
Geno Petralli set the record for allowing four passed balls in one inning while trying to catch knuckleball pitcher Charlie Hough in 1987. Varitek holds the postseason record with three in Game 5 of the 2004 American League Championship Series while catching Wakefield.
Former Atlanta Braves star Dale Murphy was a catcher all through his minor league career and entered the majors that way, but was moved to center field beginning with the 1980 season because catching teammate Phil Niekro's knuckleballs was too hard on his knees. He had several knee surgeries before then.