Codes
There are three major rules codes, which differ only slightly. The official baseball rules govern all professional play in the United States and Canada. The rules are published in book form in North America by the Sporting News, and are available online.[1] Many amateur and youth leagues use the "OBR" with only a few modifications for safety. U.S. collegiate and college-age play is often covered by the baseball rulebook of the NCAA; and U.S. high school and high school-age baseball is governed by the rules of the National Federation of State High School Associations (NFHS). Competitions operated by the International Baseball Federation (IBAF), most leagues outside North America, and youth baseball organizations such as Little League, PONY League, and Cal Ripken League all play under slightly modified versions of the Official Baseball Rules.

Differences and evolution
Unlike many other sports, the Official Baseball Rules have remained largely static during the modernization of the game. This is evidenced in much esoteric or awkward language in the rulebook; also, rules experts such as Chris Jaksa and Rick Roder have identified over 50 outright errors in the book, mostly concerning rare or subtle differences. Many baseball players, fans, and administrators view the rules and traditions of professional baseball as time-tested and nearly sacrosanct. The designated hitter rule, first introduced to play over 30 years ago in 1973, is still subject to lively debate and has been adopted by only one of the two American major leagues and one of the two Japanese major leagues though almost all other leagues around the world have adopted it.

In fact, NFHS rules allow the designated hitter to bat for any position. In contrast to the Official Baseball Rules, the NFHS is continually refining and updating its rule book to meet the needs of the United States high school game. Though the NCAA updates its rulebook less often than the NFHS, it is still more current.

However, the static nature of the rules has also been a source of the sport's appeal to many fans, as it allows reasonable comparisons to be drawn between players and teams of different eras. Such comparisons are not possible in sports in which the rules have changed significantly over the years. The static nature of the rules also allows a modern fan to easily follow an account of a game played long ago. As a result, baseball has more of a "history" than most other sports. As an illustration of this, many newspaper sports sections publish a small This Day in Baseball History feature with highlights of games played as far back as 100 years or more.

In Japan, if a regular-season game is tied after nine innings, only three extra innings may be played. The accepted codes of etiquette also differ in Japanese baseball, though these are not written in official rules. Other major differences often center around additional safety rules. Most amateur leagues prohibit malicious contact by runners, are stricter on interference calls, and require double-earflap helmets.

Gameplay

The complete Official Rules can be found at MLB.com, the official web site of Major League Baseball in the United States.z

General structure
Baseball is played between two teams of nine players each on a baseball field, under the authority of one or more officials, called umpires. There are usually four umpires in major league games; up to six (and as few as one) may officiate depending on the league and the importance of the game. There are four bases. Numbered counter-clockwise, first, second and third bases are cushions (sometimes informally referred to as bags) shaped as 15 in (38 cm) squares which are raised a short distance above the ground; together with home plate, the fourth "base," they form a square with sides of 90 ft (27.4 m) called the diamond. Home base (plate) is a pentagonal rubber slab known as simply home. The playing field is divided into three main sections:

The infield, containing the four bases, is for general defensive purposes bounded by the foul lines and within the grass line (see figure). The outfield is the grassed area beyond the infield grass line between the foul lines, and bounded by a wall or fence. Foul territory is the entire area outside the foul lines. The pitcher's mound is located in the center of the infield. It is an 18 foot (5.5 m) diameter mound of dirt no higher than 10 inches (25.4 cm). Near the center of the mound is the pitching rubber, a rubber slab positioned 60 feet 6 inches (18.4 m) from home plate. The pitcher must have one foot on the rubber at the start of every pitch to a batter, but the pitcher may leave the mound area once the ball is released. At the college/professional level, baseball is played in nine innings in which each team gets one turn to bat and try to score runs while the other pitches and defends in the field. High school baseball plays seven innings and Little League uses six inning games.

An inning is broken up into two halves in which the away team bats in the top (first) half, and the home team bats in the bottom (second) half. In baseball, the defense always has the ball — a fact that differentiates it from most other team sports. The teams switch every time the defending team gets three players of the batting team out. The winner is the team with the most runs after nine innings. If the home team is ahead after the top of the ninth, play does not continue into the bottom half. In the case of a tie, additional innings are played until one team comes out ahead at the end of an inning. If the home team takes the lead anytime during the bottom of the ninth or of the bottom any inning thereafter, play stops and the home team is declared the winner.

The basic contest is always between the pitcher for the fielding team, and a batter. The pitcher throws pitches the ball towards home plate, where the catcher for the fielding team waits (in a crouched stance) to receive it. Behind the catcher stands the home plate umpire. The batter stands in one of the batter's boxes and tries to hit the ball with a bat. The pitcher must keep one foot in contact with the top or front of the pitcher's rubber a 24" x 6" (~ 61 cm x 15 cm) plate located atop the pitcher's mound during the entire pitch, so he can only take one step backward and one forward in delivering the ball.