From BR Bullpen
The pitcher's mound (aka the hill) is the raised dirt area in the center of the infield from which the pitcher pitches. Just behind the center of the mound is the pitcher's rubber, which the pitcher must touch with his pivot foot while preparing for and making the pitch.
The pitcher's rubber is set so that its front edge is exactly 60 feet 6 inches from the rear point of home plate, and is elevated 10 inches above the rest of the playing field. The area of the mound around the pitching rubber is flat. Starting 6 inches in front of the rubber, or 60 feet from home plate, the mound slopes downward at a rate of 1 inch per foot over a span of at least 6 feet.
It is very important that the mound be maintained in good condition. The pitcher depends on having good footing on the mound, and he may be injured he slips during his delivery. This is an especially serious problem when teams are playing in a rain that isn't quite severe enough to require a rain delay.
The height of the mound has not been constant, or even well defined, through baseball history. Before 1893, the pitcher threw from a pitcher's box, which worked better with a level surface rather than a sloped one. In 1893, the pitching distance was changed, and the box was replaced with the pitcher's rubber. Pitchers discovered that they could get more speed on the ball if they were allowed to stride downhill, so their groundskeepers would provide them with a mound. Those early mounds were not regulated; in Pitching in a Pinch, Christy Mathewson commented that the height of the mound might be changed from day to day to suit the pitching style of the home team's pitcher.
The regular changing of mound height was eventually prohibited. Teams settled on a height of 15 inches for the mound. Despite this regulation, some teams were accused of using a higher than regulation height mound; Dodger Stadium was particularly notorious for having a high mound. Following the incredibly low scoring in 1968, the rules were changed to reduce the mound to the contemporary 10 inch height. Some accusations of gamesmanship with mounds continue, usually with visiting teams compaining that the mounds in the visitor's bullpen don't match the mound of the field, so that relievers entering the game aren't properly adapted to the game mound.