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The fastball is the first pitch learned and should be used more than any other pitch in the repertoire. Obviously velocity, control, and movement of the fastball dictate how often and in what situations the fastball will be used. When a pitcher is learning to throw the fastball, he should make a conscious effort to learn control and movement first, and then add velocity later. This principle applies even more for the higher-level baseball pitchers.
By slightly changing the basic fastball grips, a pitcher can get various results. Variations of the fastball are four seam, two seam, cut, and sinker. The first fastball to master is the four seam fastball. The pitcher should prove that he has good control of this pitch before he attempts to throw any others.
How to throw a four seam fastball: The four-seam fastball is the easiest pitch to control and is the first baseball pitching grip learned. The four-seamer usually doesn't have much movement, so it is an ideal pitch for young pitchers to use to master the strike zone. Because it lacks movement, this pitch is less important to the older pitcher who has good control but needs a fastball with movement. The exception to that rule is the pitcher with the great arm who can throw at around 90 mils per hour. He should use the four-seamer because the ball will have good movement at that speed --and obviously a pitcher who can throw 90 miles per hour with good control will win.
The four-seam fastball is held with the index and middle finger across the hourshoe of the baseball. The end joints of the fingers should be over the seam to ensure a good grip. The thumb should be on the bottom of the ball on an imaginary line between the two fingers on the top.
The pitcher should hold the ball loosely in the hand, and he should have at least a finger-width space between the ball and the palm of the hand. Ideally the enclosed end of the hourseshoe seam should be closest to the middle finger; this helps with the feel of the seams, given that the index finger is shorter.
The two-seam fastball has more movement because of the grip and therefore is harder to control than the four-seam fastball. It is one of the better baseball pitching grips, but often one that is never tried. The pitcher should throw the four-seam fastball until he has proven that he has mastered the strike zone. The two-seam fastball becomes more important to the older pitcher who does not have an outstanding arm and must rely more on movement than on speed. The two-seam fastball moves to the pitching-arm side of the plate. Often it will also sink, producing a pitch that tails away and down.
Place your index and middle fingers directly on top of the ball’s narrow seams. Then place your thumb directly on the bottom side of the ball on the white leather between the narrow seams.
Grip this pitch slightly farther back in the hand, as this will lead the ball to “back up” and change direction. The additional drag created from the grip generates the change in direction.
The cut fastball, or cutter, moves away from the pitcher's throwing side. A right-handed pitcher's cut fastball moves from right to left with approximately 95 percent of the velocity of the ultimate fastball. In the cut fastball grip, the thumb slides to the outside of the center line of the baseball. With the thumb slid over, the baseball is held slightly off center and therefore does not have a regular top-to-botom fastball rotation. Instead, the ball has slightly more sidespin and runs away from the pitcher's throwing side. It's a very effective pitch, just ask Mariano Rivera! But, it is often a baseball pitching grip that is never tried.
The cut fastball and the slider are similar in that both balls are held off center. The slider, however, is held more off center and therefore breaks down as well as away. The cut fastball is excellent training for the slider.
There's no better way to show this grip than from the person that throws it best, Mariano Rivera:
Baseball Pitching Grips - How to throw a sinker: The sinker moves down and to the throwing-arm side. A right-handed pitcher's sinker moves from left to right and has some down movement with approximately 95 percent of the velocity of the fastball. In the sinker grip variation, the thumb slides to the inside of the baseball's center line. Sliding the thumb up the inside of the baseball causes the ball to be held off center, giving it a side-spin that causes sinking movement. The higher the thumb goes up the side of the baseball, the more sinking movement but less velocity it has. Many pitchers slide the thumb up top and use this pitch as a change-up.