In baseball, a two-seam fastball is a variation of a fastball thrown in such a way as to impart some slight vertical or lateral motion to the pitch. Compared to the more common four-seam fastball, the two seam fastball sacrifices a bit of speed, but its slight break - usually a cutting or sinking action - is more likely to fool the batter and induce a weak ground ball or pop-up.

The two-seam fastball gets its name from the fact that the fingers only touch the seams in two places. To throw the two-seamer, the pitcher grips the ball at the point where the seams are closest together, with the seams perpendicular to the body and the index and middle fingers running along the seams. When the ball is thrown, the pitcher imparts pressure at one of various points on the ball to achieve a certain of breaking action.

A two-seam fastball that has a particularly noticeable sinking action is known as a "heavy" fastball, or a "sinker," while a two-seam fastball with a particularly noticeable lateral break is often known as a cut fastball, or "cutter."

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