ball hit over the fence on the fly is an automatic home run, which entitles the batter and all runners to touch all the bases and score. On a field with foul poles, a ball that hits a pole is also a home run. A home run hit with all bases occupied ('bases loaded') is called a grand slam. Fielding team 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 are 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). Note that, in rare cases, teams may use dramatically differing schemes, such as switching an outfielder for an infielder. The numbering convention was established by Henry Chadwick. The reason the shortstop seems out of order has to do with the way fielders positioned themselves in the early years of the game; the shortstop was positioned in the shallow outfield. Each position is weighted on the defensive spectrum in terms of difficulty. The most difficult position is the catcher, while the least difficult is first base. Designated hitter, while on the scale, is not part of the defense at all. Pitchers, while part of the active defense, are so specialized in their role that they usually make only routine plays. The battery 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", coined by Henry Chadwick and later reinforced by the implied comparison to artillery fire. The pitcher's main role is to pitch the ball toward home plate with the goal of getting the batter out.