When someone says the mound is ten inches tall, it signifies that the pitching rubber is exactly ten inches higher than the home plate surface. In order to aid surface drainage off the infield, most infields include a slope from the base of the mound down to the baselines and bases. This slope should be no steeper than 1:20 or 10%. A ten-inch-high rubber would be appropriate if the pitcher's mound was raised four feet six inches as well. However, many stadiums with twelve-inch-high mounds also have ten-inch-high rubbers, so this is not unique to baseball. A rubber that is eight inches high would be appropriate for a three-foot-six-inch pitcher since there is still room below the lip of the rubber for good defense. There are two main types of rubber surfaces used in today's games: cinder and clay. Both types of rubber are hard substances that act like ice skates on snow or ice - they help the pitcher get all his velocity out by wearing him out over time. However, clay bats tend to be heavier and more durable than cinder bats. Most major league teams use both types of bats during different parts of the season for consistency issues. During warm weather months, when pitchers can stay in longer games, they often use cinder bats because they're easier to find and less expensive than clay.