Pitches and Stuff: Two-Seam Fastball

Why does a curveball curve? How does a pitcher throw a changeup with the same arm speed as a fastball? Baseball is filled with buzzwords that are often used imprecisely. In this series of articles, Mike Richmond explores the baseball’s motion as it travels through the air: How does it behave, and what can a pitcher do to control it? This article looks at the two-seam fastball to see what causes its very distinctive break.

What effect do different grips have on pitches? What’s the difference between a cutter and a slider? The art of pitching is filled with arcane terms, and even when two players are talking about the same thing, they often use different words. In this series of articles, we’re going to look carefully at the motion of a baseball through the air: how does it behave and what can a pitcher do to control it?

This time, we look at two different varieties of the fastball, that most basic of all pitches. Not all pitches are alike, of course: each one travels at a slightly different speed and spins at a slightly different rate, in a slightly different direction. However, if one looks at the properties of pitches in bulk, one finds that most fall into just a few major groups; and the fastballs appear to be divided into two.

Evidence for two populations of fastballs

Thanks to the cameras that record the trajectory of each MLB pitch, we have many ways to view a pitcher’s repertoire. We can put ourselves at third base and watch the ball travel from mound to home plate; we can crouch down with the umpire and note the spot at which the pitch crosses the plate; or we might create a more abstract illustration by computing the differences between the motion of a spinless ball and the real one, and graphing these horizontal and vertical breaks.

But let’s try something new: a procedure designed to help us distinguish the identity of each pitch. We will put the horizontal break on the horizontal axis, and the initial speed of the ball when it left the pitcher’s hand on the vertical axis. Using the excellent Brooks Baseball site, one can retrieve this information organized in many ways. We can, for example, select a summary of LHP Jon Lester‘s work over his entire career:

There are obvious groups in this diagram. The pink bunch in the lower left are slow and break toward third base; additional information (not shown here) indicates that they have strong topspin: curveballs. The brown set in the middle of the diagram, moving around 90-mph but with little horizontal movement, are sliders. The blue cluster at moderate speed which have a break toward first base are changeups. And near the top of the diagram, in black, are the balls that move quickest: the fastballs.

But … wait a minute. Although all these fastballs appear to travel at the same speed – roughly 93-mph – they seem to have quite a large range of horizontal break values. The range is large enough that one is tempted to divide them into two groups: the four-seam (FF) (red) and two-seam (FT) (green)  fastballs:

Two types of spin

What is the difference between these two varieties of fastball? Not the initial speed; that differs by less than 1 mph. It’s the direction of the spin of the ball. All fastballs have strong backspin, but the axis of the four-seam type is close to horizontal, while that of the two-seam type is tilted slightly. In the case of Lester, the difference is about 26 degrees:

The combination of a ball’s spin and its motion through the air combine to create the Magnus force, which is perpendicular the spin axis. In this case, the Magnus force pushes a four-seam fastball straight up, but a two-seam fastball both up and slightly to the first-base side. Because a right-handed pitcher would tilt the spin axis in the other direction, the Magnus force on his two-seamer would push it toward third base. 

We can see this difference in the trajectories of two individual pitches thrown by Jon Lester: the two-seam faster shown in blue, the four-seamer in red:

Below are a pair of animations (thanks very much to Ian York), showing the combined motion of all the fastballs Jon Lester threw in 2014: four-seam in the top frame, and two-seam in the bottom frame.  Note the late movement of the two-seamers to the right (toward first base):

Two kinds of grip

What causes this difference in spin? One obvious answer might be “the way a pitcher grips the ball.” To throw a four-seam fastball, a pitcher holds the ball so that his index and middle fingers cross the seams perpendicular to one of the “horseshoes”:

Image courtesy of thecompletepitcher.com 

As the ball approaches the batter, four sets of stitches will, one after the other, appear at the bottom, crawl upward, and disappear over the top:

To throw a two-seam fastball, a pitcher holds the ball so that his index and middle fingers lie parallel to the seams:


Image courtesy of thecompletepitcher.com 

As this pitch approaches the batter, only two seams will rotate up and across the middle of the ball:

But what about the tilt?

However, the difference in the grip, by itself, does not seem sufficient to tilt the spin axis of the ball by the required twenty or thirty degrees. The ball rolls off the fingers in a very similar manner, as the nearly identical speeds would suggest.

Could there be a difference in the release point? The farther a pitcher holds his arm from the vertical when he releases the ball, the more he tilts the spin axis of the ball.  Below is a graph showing the release point of Jon Lester in one particular game in 2009.

I don’t see any significant difference in this instance, or in records for other games and other pitchers. That rules out arm angle as the reason for the two-seam’s horizontal break.

What could it be? Consider the pitcher as a machine: his arm, elbow, wrist, hand and fingers all work together to fling the ball toward home plate. We have ruled out the fingers and the arm; my guess is that the wrist may be the best remaining suspect. If one not only grips a two-seam fastball differently, but also snaps the wrist slightly just before the release, one might impart a small tilt in the spin axis.

If you know the answer to this question, please let us know! Use the comments to share your wisdom with us all.

The difference can be subtle

Not all pitchers throw the two-seam fastball in the same way. Jon Lester’s method produces a big difference between his four-seam and two-seam varieties: a gap of nearly five inches in horizontal break. On the other hand, when Francisco Liriano is on the mound, the gap between the pitches is almost non-existent:

Another pitcher whose two-seamer looks very much like his four-seamer is Jake Peavy. Peavy throws at least six different pitches, but his fastballs look very much alike:

The difference between two-seam and four-seam pitches varies from one pitcher to the next; the one common factor is that the two-seam version always has a slightly larger horizontal break to the pitcher’s hand side.

When and how to use two-seamers

Since two-seam fastballs do move more towards the pitcher’s hand side — towards third base for a right-handed hurler, or towards first base for a lefty — does that mean that they will be more effective against certain batters?

Well, one aspect of the pitcher-batter battle is that, in general, left-handed batters have more trouble with left-handed pitchers, and vice versa. Managers will often bring in relievers specifically to turn this “righty-lefty” factor to their advantage. One explanation for this effect is that a right-handed pitcher’s curveball will naturally break away from a right-handed batter, and balls which break away from a batter are just harder to hit.

If we accept this premise, then a pitcher should preferentially throw two-seam fastballs when they break horizontally away from a batter. Since fastballs with their backspin break in the direction opposite to topspin-heavy curveballs, that means a right-handed pitcher should throw two-seamers to a LEFT-handed batter.

Jered Weaver is a right-handed pitcher with a decent gap between his four-seam and two-seam fastballs:

Does he throw the two-seamer preferentially to left-handed batters? In a game against the Tampa Bay Rays on August 24, 2014, he certainly did! He not only threw many more two-seamers to left-handed batters, he also aimed them for the outer part of the plate:

By the same argument, a left-handed pitcher ought to throw his two-seamer more frequently to RIGHT-handed batters. Let’s go back to Jon Lester’s game against Seattle on July 5, 2009:

Contrary to our hypothesis, the numbers here are roughly equal: he threw 13 four-seamers to RHB and 18 to LHB, while he offered 14 two-seamers to RHB and 18 to LHB. There is no excess of two-seam fastballs thrown to right-handed batters. Like Weaver, he located most of his two-seam fastballs in the outer half of the plate against left-handed batters, even though Lester’s two-seamer was breaking back toward the hitter.  Rather than hoping that the batter would chase a ball fading away from him and out of the strike zone, Lester tried to convince the batter that this pitch was too far outside to bother swinging, only to fool him when it swerved into the strike zone “through the back door.”

It would appear that pitching involves much more than one or two simple rules applied without variation.

Why is the two-seam fastball sometimes called the “sinker?”

I have no idea. Like its four-seam brother, the two-seam fastball has a positive vertical break. In other words, compared to a ball thrown with no spin, it ends up higher in the strike zone. To illustrate this fact, I will make graphs showing the vertical and horizontal break of different pitches. In these diagrams, a ball with no spin would end up in the very middle, at (0, 0). 

Jered Weaver’s two-seamer has the same vertical motion as his four-seam fastball:

Note how the symbols for the two-seam fastball mingle together with those for the changeup.   This isn’t peculiar to Weaver; as others have noted, the only real difference between a two-seam fastball and a changeup is the extra 7-10 mph.

Jon Lester’s two-seamer is no exception, though it has quite a range of vertical break. In every case, though, the “sinker” ends up far above the location of a hypothetical ball with no spin:

So why call the two-seam fastball a “sinker?”  This is another puzzle that I can not solve. If you know the answer, please provide it in the comments.

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About Mike Richmond 12 Articles
Michael grew up on the South Shore of Massachusetts, but rebelled against his parents by rooting for the Orioles (eventually, he came to his senses). After receiving his Ph.D. in Astronomy from UC Berkeley, he spent five years as a post-doc at Princeton working on the Sloan Digital Sky Survey. He now lives in Rochester, NY, studying supernovae and listening to baseball games far too often.

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