The starting point for much of the action on the field is home plate (officially "home base"), which is a five-sided slab of whitened rubber, 17 inches (43 cm) square with two of the corners removed so that one edge is 17 inches long, two adjacent sides are 8.5 inches (22 cm) and the remaining two sides are 12 inches (30 cm) and set at an angle to make a point. The plate is set into the ground such that its surface is level with the surrounding ground. Adjacent to each of the two parallel 8.5-inch sides is a batter's box. The point of home plate where the two 12-inch sides meet at right angles is at one corner of a 90-foot (27.43 m) square. The other three corners of the square, in counterclockwise order from home plate, are called first, second, and third base. Three canvas or rubber bases 15 inches (38 cm) square and 3–5 inches (7.6–12.7 cm) in thickness made of soft material mark the three bases.
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Near the center of the square is an artificial hill known as the pitcher's mound, atop which is a white rubber slab known as the pitcher's plate, colloquially the "rubber." The specifications for the pitcher's mound are described below.
All the bases, including home plate, lie entirely within fair territory. Thus, any batted ball that touches those bases must necessarily be in fair territory. While the first and third base bags are placed so that they lie inside the 90-foot square formed by the bases, the second base bag is placed so that its center (unlike first, third and home) coincides exactly with the "point" of the ninety-foot square. Thus, although the "points" of the bases are 90 feet apart, the physical distance between each successive pair of base markers is closer to 88 feet (26.8 m).
The lines from home plate to first and third bases extend to the nearest fence, stand or other obstruction and are called the foul lines. The portion of the playing field between (and including) the foul lines is fair territory; the rest is "foul territory." The area within the square formed by the bases is officially called the infield, though colloquially this term also includes fair territory in the vicinity of the square; fair territory outside the infield is known as the outfield. Most baseball fields are enclosed with a fence that marks the outer edge of the outfield. The fence is usually set at a distance ranging from 300 to 420 feet (90 to 130 m) from home plate. Most professional and college baseball fields have a right and left foul pole which are about 440 to 500 feet (130 to 150 m) apart. These poles are at the intersection of the foul lines and the respective ends of the outfield fence and, unless otherwise specified within the ground rules, lie in fair territory. Thus, a batted ball that passes over the outfield wall in flight and touches the foul pole is a fair ball and the batter is awarded a home run.
Citation
"Official Rules: 1.00 Objectives of the Game". Major League Baseball. Retrieved 26 October 2014.
"DIAGRAM NO 2". Official Rules of Major League Baseball, 2013 Edition. Triumph Books. 2013. p. 5. ISBN 978-1-60078-797-3.
http://web.mit.edu/~xsdg/Public/papers/himcm-2003.pdf "The width is the distance between foul poles... the Twins field width (473.9 ft) and the Braves field width (470.2 ft) is not significant. However, the difference between the Rockies and Yankees field widths (492.9 ft and 446.9 ft, respectively) is very significant."
