From its invention in 1887 as an indoor game, softball grew rapidly in the United States and Canada through the early part of the twentieth century. Fast-pitch softball had its rules codified in 1933, and the game acquired an international following, precipitating the formation of the International Softball Federation (ISF) in 1952. By the time of the 1996 Summer Olympics, when women's softball made its debut as an official Olympic competition, the ISF had approximately 90 member nations.
Slow-pitch softball, also known as "lob ball," also grew in popularity across the United States beginning in the early 1950s. The manner in which the ball is delivered by the pitcher is the key distinguishing feature between the two types of softball. Where the fast-pitch game depends on a powerful pitcher who can deliver the ball with either great velocity or with deceptive ball movement, slow-pitch encourages the batter to hit the ball, put the ball in play, and force the defensive team to make strong fielding plays to generate outs, as opposed to strikeouts by the pitcher.
There is a third variant of softball, modified pitch softball, that has a more limited following. Played with nine players in the field, the ball must be delivered by the pitcher underhanded in a prescribed manner, similar to slow-pitch.
In slow-pitch, the pitcher must deliver the ball underhanded, with a minimum arc of 6 ft (1.8 m) and a maximum arc of 12 ft (3.6 m). The umpire has the discretion to rule a pitch as illegal due to insufficient or excessive arc. Most pitchers attempt to deliver the ball in such a fashion that the ball is dropping in an arc that is as close to perpendicular to the ground as possible. The closer to perpendicular the path of the ball, the more the hitter will be inclined to swing at the ball with an uppercut, as opposed to a level swing. When the player swings with an uppercut stroke, the player tends to use only the arms and the shoulder muscles, limiting the power and the speed with which the bat will strike the ball. An uppercut stroke is also less likely to make maximum contact between the bat and the ball surface.
When the ball is delivered with a flatter arc, a capable hitter will swing with a more level stroke, striding forward from the plate, driving the ball with both arms and shoulders, as well as the torque generated by the twist of the batter's torso, hips, and legs. A poorly thrown slow-pitch ball will often be delivered for a homerun. Prodigious homerun hitters in slow-pitch softball take advantage of two physical principles inherent in the batter's stroke: the fact that an aluminum bat can be swung faster, thus generating greater bat force upon the ball, and the "trampoline effect," which is the physical reaction of the ball when it makes contact with the aluminum bat barrel.
Slow-pitch softball and fast-pitch softball are played on fields with similar dimensions. In addition
Fast-pitch softball is played with virtually all of the rules of traditional baseball and therefore employs very similar strategies. A strategy common to both games is the use of the bunt, the deliberately restricted swing and contact with a pitch by the batter to place the ball in the infield to either permit the batter to reach first base or to advance a teammate as a "sacrifice." The bunt is not permitted in slow-pitch; it is umpire's discretion as to rule a poorly hit ball that rolls a short distance in front of home plate as intentionally played.
Another tactical limitation placed upon slow-pitch is the prohibition against base stealing, a strategy available in both fast-pitch and baseball. The rationale for this limit is the fact that the offensive team has better opportunities to advance a runner through the batter.
The most striking difference between fast-pitch and slow-pitch softball may demographical. Through the removal of the high-speed offering from the pitcher to the batter, slow-pitch by definition is both a safer and an easier game to play. It is very popular in recreational leagues throughout North America for this reason; it is one the few sports that enjoys significant popularity as a mixed gender sport, both recreationally and competitively. Slow-pitch is also well suited to age group competition.