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). 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.