Forkball pitchers are a rare breed, perhaps even rarer than the elusive knuckleballer.

There are two regular knuckleball pitchers currently starting in the major leagues: Tim Wakefield and R.A. Dickey. Charlie Haeger has flipped the knuckler in a few big-league games, Charlie Zink has an even shorter cup of coffee, and pitchers like Josh Banks and Eddie Bonine have made rare use of the knuckleball as part of a larger repertoire. The recent roster of knuckleballers is rich but short.

The list of recent major-league forkballers is shorter—a mere three names long: Jose Contreras, Scott Linebrink, and Justin Speier.

The forkball is a pitch that isn’t used much anymore in the major leagues, though its cousin, the split-finger fastball, is a popular pitch. What is the difference between the forkball and the splitter? The Neyer/James Guide to Pitchers says the following:

Precursor to the split-finger fastball, the forkball was popularized shortly after World War I by Bullet Joe Bush, then with the Red Sox. Basically a sort of change-up, Bush's forkball was gripped with the index and middle finger spread as far apart as possible, and when thrown correctly would travel plateward with little spin, almost like a knuckleball, and appear to dip suddenly upon its arrival to the plate.

Over the years, it's likely the pitch was generally thrown with the fingers held closer together, and more spin.

What distinguishes the forkball from the splitter? As thrown by Bush and Roy Face (its most famous practitioners), the forkball is held deeper in the hand and thrown with less velocity than the split-finger fastball (think of the hard curve vs. the slow curve).

The forkball is a pitch whose history goes back to the early part of the last century. The splitter is a newer development. According to the Neyer/James history, the splitter was “invented by Fred Martin, perfected by Bruce Sutter in the late 1970s, and popularized by Roger Craig in the early '80s.”

The similarities and the differences between the two pitches begin with the grip. In both cases, the pitcher holds the baseball between his index finger and middle finger. With the forkball, the ball is wedged as deeply as possible between the two fingers and flipped out of the hand without much spin or force from the fingers. This results in a slow, tumbling pitch.

The forkball grip apparently is not an easy one on the pitcher’s hand. Jim Salisbury wrote that Contreras keeps a softball handy to keep his index and middle fingers stretched out.

Pitchers use a variety of grips for the splitter, but for a typical splitter, the ball is not wedged as deeply between the fingers so that the pitcher can provide either some finger force behind the ball, or some sidespin off the middle finger when pronating on release, or some of both. This results in a sinking pitch that spins and looks like a fastball but comes in slower to the plate and drops more than the heater.

Major-league pitchers throw the splitter between four and 11 mph slower than their fastball, with the average between seven and eight mph slower. That means that on the slow end, the splitter acts like a regular sinking changeup, and on the faster end, like a hard-sinking changeup or slow-sinking fastball.

The forkball, on the other hand, pitches its tent firmly in the camp with off-speed pitches. Speier’s forkball clocked in as the hardest of the bunch at nine mph slower than his fastball. The forkballs from Contreras and Linebrink were both about 13 mph slower than their fastballs. That speed differential is on the very slowest fringe of changeup territory and would fit comfortably among curveballs.

Another prominent feature of the forkball is its low spin rate. The knuckleball, of course, is the king of slow spin rates. A good knuckler turns less than half a revolution during its half-second trip to the plate. Typical major-league pitches (i.e., fastballs, curveballs, sliders, and changeups), spin at about 2,000-2,500 revolutions per minute (rpm). That means that the baseball makes about 15 revolutions on its way from the pitcher’s hand to the plate, depending on the speed of the pitch and the exact spin rate. A forkball rotates at about half that rate, making roughly nine revolutions from release to plate.

According to interviews of Kirby Puckett and Wade Boggs conducted by Bill Koenig, the forkball’s spin has a distinctive tumbling appearance.

[A]s the ball rotates toward the plate – usually 14 to 16 revolutions per pitch – the best hitters can ''read the seams'' to learn the pitch's identity. The spinning seams create distinctive, tell-tale patterns.

A slider appears to have a red dot, often near the ''2 o'clock'' position on the ball. A curveball gives the ball a tumbling effect, with horizontal stripes rolling downward…

A fastball looks very white and clear, with the seams barely visible. A four-seam fastball is slightly darker than a two-seamer…

A forkball also has a tumbling effect, but it appears more oblong than a curveball.

Forkball spin differs from splitter spin in an additional way, in the orientation of its spin axis.

Splitters, like changeups and sinking fastballs, have some backspin that comes from the ball snapping or rolling off the pitcher’s fingers. Splitters, changeups, and sinkers also have sidespin from the pitcher turning the ball over with his middle finger against the ball as he pronates just prior to release. The backspin produces an upward force on the ball that partially counteracts the drop from gravity. Splitters and changeups usually have less backspin than fastballs, and they are thrown slower than fastballs, so gravity has longer to act to pull the ball down and it is countered by less backspin force. The sidespin produces tailing arm-side movement.

The three forkballers for whom there is PITCHf/x data all threw their forkballs with topspin, like a curveball, rather than backspin like a splitter or a changeup. This implies that they must be supinating on release—turning the wrist and forearm so that the palm faces up and toward the body after release. Topspin makes a forkball and a curveball drop more than they normally would due to gravity alone.

Adam Whitehouse described Linebrink’s forkball thusly:

This forkball travels at around 80 MPH and tumbles downward with little rotation, after initially popping up out of Linebrink's hand. Essentially it acts like a curveball, but doesn't spin like a curveball, and in turn it makes many hitters return to the dugout scratching their heads.

Because of the low spin rate, the forkball doesn’t drop as much as a typical curveball, and it doesn’t usually have much left-to-right spin deflection, either.

Let’s look at the detailed pitch data from PITCHf/x for the forkball pitchers, starting with Contreras.

Contreras threw his forkball at an average speed of about 81 mph in 2010, 13 mph slower than his 94 mph fastball. Speed-wise, the gap between his fastball and forkball was similar to that of a very slow changeup or a curveball. Contreras does get some glove-side movement on his forkball, similar to but somewhat less than one would expect from a changeup because of the low spin rate on the forkball. You can see some photos of his forkball grip here and here.

What differentiates the forkball from a changeup is the amount of drop on the pitch. In addition, there is a great deal of variability in how much the pitch drops. Contreras’ forkball can drop anywhere from 18 inches, like a typical changeup, to nearly three feet, similar to a big curveball.

If the effect of gravity is removed, we can look at the effect of the spin force alone on the baseball.

Looking at spin by itself shows that Contreras must have put topspin on most of his forkballs, resulting in a negative spin deflection. This means that his forkball dropped more than it would have due to gravity alone. By comparison, a typical major-league curveball has about six inches of negative spin deflection and about five inches of glove-side movement from spin. Contreras’ forkball has similar drop to a curveball, but moves about four inches to his arm side instead.

Scott Linebrink’s forkball is similar to the one Contreras throws in some ways. It’s also a slow tumbler, but its movement is more reminiscent of a curveball than the Contreras forkball.

Linebrink threw his forkball at an average speed of 82 mph in 2010, almost 13 mph slower than his 95 mph fastball. His forkball didn’t move much to the arm side. In fact, it had very little left-right spin deflection and was somewhat more likely to move an inch or two to the glove side. You can see examples of Linebrink’s forkball grip here and here.

Speier’s forkball movement fell somewhere between that from Contreras and Linebrink, although his was thrown somewhat harder relative to his fastball speed.

Speier threw his forkball at an average speed of about 81 mph, about nine mph slower than his 90 mph fastball. His forkball had, on average, a couple inches of arm-side movement. Photos of Speier’s grip are available here and here.

That covers the three recent forkballers and the characteristics of their pitches, but the question may arise: what about all the other pitchers who are said to throw the forkball? Do they?

It is difficult to differentiate pitch types based on the photos of grips alone. Many pitchers split their fingers wide around the baseball. It’s hard to tell, though, from photos alone exactly how deep the ball is wedged between the fingers and whether the pitcher is flipping the ball out with low spin, rolling the ball off his fingers for backspin, or supinating to get topspin.

The PITCHf/x data indicates that Contreras, Linebrink, and Speier are the only three major leaguers that regularly threw a slow pitch with topspin and a low spin rate. Several other pitchers threw pitches with one or two of these fundamental forkball characteristics, but not all three.

For example, Brandon League threw a slow pitch, about 10 mph slower than his fastball, which had some topspin. However, his low arm angle likely contributed more to the topspin than any supination on release, and his pitch had a much higher spin rate than a typical forkball, producing about eight inches of arm-side spin deflection on average. Similarly, Matt DeSalvo threw a pitch about 13 mph slower than his fastball with a couple inches of drop due to topspin. However, DeSalvo’s pitch had a very high spin rate, resulting in an amazing 11 inches of arm-side spin movement.

Dan Haren and Hiroki Kuroda threw pitches with low spin rates, resulting in some tumbling action. However, both of them threw their pitches much harder than typical forkballs, only five or six mph off the fastball. Also, the splitters from Haren and Kuroda exhibit a few inches of upward spin deflection due to backspin. Tim Lincecum’s splitter, which he calls a changeup, is a similar pitch that is thrown about eight mph slower than his fastball.

The splitters thrown by Jose Valverde and Keiichi Yabu probably came the closest to qualifying as forkballs. Both of their splitters have very low spin rates. However, they are thrown at more typical splitter speeds, six or seven mph slower than their fastballs. In addition, though they don’t have much backspin, they also don’t have any topspin. As such, their splitters don’t really belong with the much slower classic forkballs with topspin. Here is a photo of Yabu’s grip and a detailed discussion of Valverde’s various pitch grips, including his splitter.

Rich Harden has an interesting pitch that is worth mentioning in this context. He uses a grip that looks quite similar to a circle changeup. However, unlike the typical arm-side sinking and tailing action of a circle change, his pitch has a very low spin rate and very little left-right spin deflection. If it weren’t for the evidence about the grip Harden uses, his pitch could easily be lumped in with splitters like Kuroda’s and Lincecum’s.

The terms splitter and forkball are often used interchangeably to describe a pitch where the index and middle fingers are split around the baseball in any fashion. This seems to be especially true in descriptions of Japanese players. Nonetheless, very few pitchers actually throw the slow, tumbling, dropping forkball.

Speier has already departed from the major-league ranks, and Contreras recently turned 39 for at least the first time. Scott Linebrink is the spring chicken of the bunch at age 34. When Linebrink and Contreras retire, will there be anyone to take up the mantle of the true forkballers?

 Not to mention, what does it take for the forkballers to get a club as legendary and mysterious as the knuckleball club?