The most notable difference between Chicago softball and standard softball is the size of the ball. The Chicago softball is 16 inches in diameter, compared with the 12-inch standard-size softball. The ball is huge by comparison, and those who have played a lot of 12-inch softball have a tough time making the switch. It's difficult to throw and it can be difficult to hit for distance.
Fielders are not allowed to wear gloves while playing Chicago-style softball. While the ball is large and can be caught with two hands, playing barehanded will punish infielders who have line drives hit at them in the first two innings. That's because the ball is very hard the first two innings or so, but once it gets hit around for a while, it softens up. That's of little comfort to a first baseman who has to catch a steaming throw from a shortstop who is trying to throw the batter out at first. Many players will suffer hand and finger injuries as a result of catching hard line drives or throws from infielders.
Pitching in Chicago-style softball is also quite different than standard softball. Instead of pitching from the mound or the rubber, the pitchers is allowed to take a sideways or backward jump off the rubber and pitch from where he lands. He cannot be closer than the pitching rubber, but he can create a different angle to pitch from in order to confuse the batter. The pitch is a high-arching pitch that will be called a strike if some part of the ball crosses the plate between the batter's shoulders and knees. If the pitch is over the batter's head but drops sharply so a small part of the ball dips to shoulder level at the rear portion of the plate, the pitch is called a strike. This will occasionally cause problems for hitters, but the game is primarily noted for players who can drive the big ball tremendous distances.
A batter gets only two strikes in Chicago-style softball. If the first pitch is a called or swinging strike and the second pitch is fouled off, the player has struck out. You only get two opportunities to swing the bat, and if you can't put it in play by the second swing, you are out. If a pitcher throws three balls, it's a base on balls.
Players are not allowed to steal bases but they are allowed to take leads. Many players will jump off first or second base and stray quite a distance because they would like to induce a pickoff throw from the catcher. If the catcher does try to pick off the runner, he can advance to the next base. However, if the catcher ignores the runner, the player must return to his original base.