The squad in the field is the defensive team; they attempt to prevent the baserunners from scoring. There are nine defensive positions, but only two have a mandatory location (pitcher and catcher). The locations of the other seven fielders is not specified by the rules, except that at the moment the pitch is delivered they must be positioned in fair territory and not in the space between the pitcher and the catcher. These fielders often shift their positioning in response to specific batters or game situations, and they may exchange positions with one another at any time.
The nine positions most commonly used (with the number scorekeepers use) are: Pitcher (1), Catcher (2), First Baseman (3), Second Baseman (4), Third Baseman (5), Shortstop (6), Left Fielder (7), Center Fielder (8), and Right Fielder (9).
The battery is composed of the Pitcher, who stands on the rubber of the mound, which is also known as the pitching plate, and the Catcher, who squats behind home plate. These are the two fielders who always deal directly with the batter on every pitch, hence the term "battery".
The pitcher's main role is to pitch the ball toward home plate with the goal of getting the batter out. Pitchers also play defense by fielding batted balls, covering bases (for a potential tag out or force out on an approaching runner), or backing up throws. The catcher's main role is to receive the pitch if the batter does not hit it. Together with the pitcher and coaches, the catcher plots game strategy by suggesting different pitches and by shifting the starting positions of the other fielders. Catchers are also responsible for defense in the area near home plate.
The four infielders are the First Baseman, Second Baseman, Shortstop, and Third Baseman. Originally the first, second and third basemen played very near their respective bases, and the shortstop generally played "in" (hence the term), covering the area between second, third, and the pitchers box, or wherever the game situation required. As the game evolved, the fielding positions changed to the now-familiar "umbrella", with the first and third baseman generally positioned a short distance toward second base from their bases, the second baseman to the right side of second base standing farther away from the base than any other infielder, and the shortstop playing to the left of second base, as seen from the batter's perspective.
The three outfielders, Left Fielder, Center Fielder, and Right Fielder, are so named from the catcher's perspective looking out onto the field. The right fielder generally has the strongest arm of all the outfielders due to the need to make throws on runners attempting to take third base. The center fielder has more territory to cover than the corner outfielders, so this player must be quick and agile with a strong arm to throw balls in to the infield; as with the shortstop, teams tend to emphasize defense at this position. Also, the center fielder is considered the outfield leader, and left- and right-fielders often cede to his direction when fielding fly balls. Of all outfielders, the left fielder often has the weakest arm, as they generally do not need to throw the ball as far in order to prevent the advance of any baserunners. The left fielder still requires good fielding and catching skills, and tends to receive more balls than the right fielder due to the fact that right-handed hitters, who are much more common, tend to "pull" the ball into left field. Each outfielder runs to "back up" a nearby outfielder who attempts to field a ball hit near both their positions. Outfielders also run to back up infielders on batted balls and thrown balls, including pick-off attempts from the pitcher or from the catcher.