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From BR Bullpen
A ground ball or grounder is a ball hit by a batter towards the ground. It will either bounce or roll towards the outfield. A ground ball is distinguished from a fly ball, which is hit with a high arching trajectory, and a line drive, which is hit on a relatively straight line. A ground ball is usually the result of contact made by the bat on the upper part of a pitched ball, or a ball hit with a descending swing. Ground balls are relatively easy to field if they are hit in the general direction of a fielder, and will often result in a double play if hit with a runner on first base. However, there is an inherent risk that a ground ball will take a bad bounce when it touches a natural surface, or that it will be slowed so much by friction that a fast batter will reach first base before the ball can be fielded and relayed there. A ball hit sharply downward may result in a Baltimore chop, a ball that bounces so high that no fielder can make a play on it in time to record the out at first. Batters who are fast runners but lack power are told to hit the ball on the ground to take advantage of these difficulties.
Pitches that result in a large number of ground balls are those with a sinking effect such as a sinker, a splitter or a forkball. Relief pitchers who induce a lot of ground balls can be very useful when a team is trying to get out of a jam with men on base and less than two outs.
A ground ball that goes through the infield will usually result in a single, although if it is hit down the third or first base foul line, it will produce a double or even more. There have been a few rare cases when a particularly well placed ground ball has resulted in an inside the park home run, although this is a rarity. A ground ball hit just beyond the reach of a fielder, or exactly between two infielders, is known as a seeing-eye ground ball, as if the ball had seen the oncoming fielder and modified its course in order to escape his grasp.