Baseball pitches aren’t living creatures; they don’t have feathers. They do fly, though, and to an incurious or uninitiated observer, their flights and behaviors could blend together. “Pitches,” one might declare them, just as Charles Darwin might've gazed upon “birds” in the Galapagos Islands and moved along.

Instead, his mind occupied by relatively new classification systems for nature and geology, Darwin collected a range of what he thought were finches to investigate their seemingly minute differences and discovered more than a dozen distinct species. Having sorted them, he moved on to drawing lines among them, explaining the differences and eventually putting forth the theory of evolution.

In recent seasons, MLB’s fundamental flying species have sparked new interest. New — or newly labeled — pitches have awakened insiders and fans to the idea that baseball’s Kingdom of Stuff, as pitcher arsenals are bluntly, hilariously known, is not preordained or set in stone. For example, a list of pitch types that had remained mostly static for decades — fastball, slider, curveball, changeup — has expanded to include the sweeper.

As we collect and analyze more information about the intricacies of each pitcher’s offerings and learn more about the increasingly scientific art of designing them, understanding pitch classifications can unlock a new appreciation for the game and the ways it’s changing.

And it is changing. Not in an “old man yells at cloud” way — in an unstoppable, “no one sends faxes anymore” way. Pitchers are comparing pitch design notes in clubhouses. Hitters are discussing the shape of breaking balls. Following baseball in 2023 means following the quest for the biggest, brightest, boldest pitches. It means following a constant evolution.

Below, I’ll break down each of MLB’s well-defined pitch types by sight, by purpose and by example. The pitches are grouped into families — fastballs, breaking balls and offspeed pitches — and in between, I’ll touch on some of the big questions that arise from the task of classification.

First up, the fastballs:

What is a four-seam fastball?

The bread and butter. What Bruce Springsteen probably meant when he sang about “that speedball” in “Glory Days.” The go-to — at least for now.

What does it look like? MLB’s most common pitch, this is the overpowering heater of movies, dreams and GIFs. Four-seams, which average 94 mph across baseball, are relatively straight, typically running slightly to the pitcher’s arm side. They are designed to defy gravity via their intense backspin, creating the illusion of “rise.”

Why do pitchers throw it? For all sorts of reasons. The primary offering for many hurlers, the four-seam is thrown all over and in all sorts of counts but is most effective when located up in the zone or above it. Because it stays up more than hitters’ brains expect, a good four-seam often flies over the top of the bat. It is the best fastball for getting a swing-and-miss or a pop-up — which has, in turn, made it the No. 1 choice in an era when numbers prove the superiority of those two outcomes.

Who has a good one? Justin Verlander’s four-seam might be the best fastball of the generation, all the more impressive because he achieves it with such an imposing, over-the-top motion. A newer platonic ideal of the form — literally created in a lab — belongs to Atlanta Braves star Spencer Strider. It ticks all the boxes: high-end velocity, lots of spin to ward off gravity and an especially flat trajectory that evades bats. Luis Castillo, Zac Gallen and Joe Ryan also hurl elite four-seamers.

What is a sinker/two-seam fastball?

Bewitched bat-breaker. An invitation to swing, batter batter, swing. The worm-burner.

What does it look like? Tailored to run hard toward the pitcher’s arm side and often drop, this is the pitch used to miss barrels. A primary pitch for some and a complementary notch in other starters’ array of fastballs, it is often thrown low in the zone. Averaging 93.3 mph this season across MLB, the sinker bores in on same-handed hitters and away from opposite-handed hitters.

Why do pitchers throw it? While some especially wicked sinkers can miss bats at prolific rates, the more common goal is to induce weak contact, typically on the ground. This pitch’s trademark horizontal movement helps throw swings slightly off-course, to where the ball connects with the less propulsive bat handle or cues off the end. Because it veers in an unnatural-seeming direction, the sinker can also be very effective at stealing called strikes via “back door” or “front door” action.

Who has a good one? Framber Valdez, Logan Webb and Marcus Stroman rank among the most prodigious sinkerballers. NL Cy Young winner Sandy Alcantara is one of the pitchers with the dexterity to mix sinkers and four-seamers roughly evenly. But the most visually startling example of the form belongs to New York Yankees reliever Clay Holmes.

What is a cutter?

A dart. Hitting the fairway on a dogleg hole. The slider-fastball midpoint.

What does it look like? A cutter is a fastball with a hint of a slider’s bite. It flies either straight or slightly to the pitcher’s glove side, where the other types of fastballs tend to move to the arm side. Depending on the pitcher, it can look like an angular, loping heater or a particularly firm slider. On the whole, cutters go a few ticks slower than fastballs, averaging 89 mph.

Why do pitchers throw it? The only pitch Mariano Rivera needed can be a singular and diabolical weapon. However, the pitch is more often a complementary option alongside other fastballs. For starters who need modes of attacking hitters multiple times and working efficiently, a cutter can inspire doubt or disrupt balance enough to help other pitches play up. Those using it as one member of an ensemble cast are likely to feature it much more often against opposite-handed hitters, to whom it will break in on their hands.

Who has a good one? 2021 NL Cy Young winner Corbin Burnes uses his as a primary fastball, and relievers such as Kenley Jansen and Emmanuel Clase have built careers on the backs of excellent cutters. Cutters are also targeted weapons in the middle of broader arsenals. Max Scherzer has long wielded an effective one sandwiched between an excellent four-seam and an excellent slider. Lefties such as Nestor Cortes Jr. and Eduardo Rodriguez deploy theirs liberally against right-handers. And plenty of pitchers who added a sweeper in recent seasons use the cutter as its more tightly wound sparring partner.

Are there actually more pitches now?

In 2011, Zack Wheeler watched a video of Mariano Rivera describing how he held his cutter and decided to start throwing the pitch in games at High-A San Jose, where he was a 21-year-old prospect in the Giants system.

“It turned into a slider one game, and I was like, ‘I want that,’” Wheeler recalled recently, a dozen years on.

In 2023, this sort of experimentation comes with the support of a full-fledged suite of technology, data and instruction. Wheeler, now 33 and a star with the Philadelphia Phillies, said he learns a new pitch every offseason, adding options and beating back predictability.

Wheeler's approach might be particularly proactive, but his interest in The Next Pitch is widely shared. Pitchers across the league today have become fluent in the movement metrics that help define pitches, the detailed statistics that quickly gauge their performance and the language of pitch design that aids in their development.

“The pitch tags and classifications are just based upon movement, which is fine,” said Tyler Zombro, a Tread Athletics instructor who helps guide hurlers through the increasingly detailed process of building and revamping their arsenals. “But I'm actually much more intrigued by how said pitcher thinks about each pitch.”

The difference between a pitcher throwing a cutter or a slider or a sweeper is a question not of semantics but of intent. He might want to throw them all and ensure they maintain separate profiles for separate purposes.

Per research from Baseball Prospectus’ Pitch Info unit, the average MLB starter throws 4.23 different pitches at least 5% of the time. That average hasn’t changed much over the past decade, but the path to it has, as more pitchers are moving toward the extremes.

Almost a quarter of starters now use narrow, two- or three-pitch arsenals typically characterized by overpowering stuff. A rising portion, though, are using wide arsenals of six or more pitches. (In this data, we’re looking at pitchers with a minimum of 120 innings in full seasons, or 40 innings in 2020 and 2023.)

While we can’t leap to any huge conclusions from that data, it’s safe to say that more pitchers — those who perhaps don’t have the high-octane stuff of a Spencer Strider — are exploring more avenues because the resources to do so are at their disposal.

“It's nice to have weapons versus all handedness and bat paths so it gives you more options to play with,” said Dean Kremer, a Baltimore Orioles starter and one of the 12 pitchers carrying six or more pitches in 2023, having added a sweeper over the winter.

These are the sorts of things pitchers are considering now. It’s not just lefty or righty. It’s about how the batter’s swing might play against each pitch. It’s also about what their own natural movements put them in the best position to throw.

“I think a lot of it,” said Chris Langin, Driveline Baseball’s director of pitching, “comes down to just understanding the purpose of the pitch or why a pitcher would need it.”

At Driveline, an influential facility that routinely helps major-leaguers add new pitches, there are layers and layers of knowledge below the surface of a single pitch classification, including several versions of grips and processes that trainers match to pitchers based on their individual characteristics, then evaluate with pitch-tracking technology and high-speed cameras. At some point, the entire thing goes back to the tactile, tangible moment of truth: how to hold and throw the ball.

Wheeler, reflecting on how much more accessible pitching information has become, stopped to praise Phillies pitching coach Caleb Cotham for his thorough knowledge of spin.

“He really understands that. He's good at explaining it. I'm more of a visual guy, so he'll sit there and spin the ball in his fingers,” Wheeler said, demonstrating how Cotham creates a visual display of the baseball spinning on an axis, almost like a classroom globe.

Wheeler said his slider, which was originally a cutter based off Rivera’s, has gained consistency because of Cotham and the heightened attention to detail — even as he has also added the trendy sweeper into his mix.

Langin predicts that some of the fervor around adding more and more pitches will subside and a more focused approach will reign, but Wheeler? He’s already thinking about what he might try this offseason — and hopes more guys will feel confident enough to experiment.

“It's 10 things they have to think about,” he said, “instead of two or three.”

Next up, the most fertile ground for that: breaking balls.

What is a slider?

Now you see it, now you don’t. Baseball’s ankle-breaking crossover. The knockout punch.

What does it look like? Ideally, it looks like a fastball right up until the moment it dives and/or darts to the glove side. A slider is thrown harder than other breaking balls, averaging 85 mph, but it relies on the pitcher's release to create a combination of side spin and bullet or “gyro” spin to get snapping, swerving movement in the middle of its flight.

[Read more: Could sliders one day surpass fastballs as baseball’s primary pitch?]

Why do pitchers throw it? This is the sport’s preeminent swing-and-miss pitch, and it also tends to produce grounders because of its downward tilt. Often, pitchers try to start a slider in the same tunnel, or typical flight path, as their fastball in order to bait hitters to swing at a pitch that is actually going slower and in a different direction than anticipated. The slider’s usage trend line is steadily going up as more pitchers deem it their best chance to get outs.

Who has a good one? Power pitchers such as Gerrit Cole, Verlander and Scherzer employ terrific sliders that play off their fastballs. Others such as Clayton Kershaw have turned to the slider as an equal costar or leading player. A barrage of relievers from Edwin Diaz to Matt Brash also lean on their breathtaking sliders to create MLB’s version of posterizing dunks: strikeout GIFs for Twitter.

At the top of the slider game, though, velocity rules — testing the limits of human physicality and asking hitters to adjust to an object bending its path in midair at 92 mph. Which is how Jacob deGrom — when healthy — dominates by relentlessly throwing his slider to the exact same spot.

What is a sweeper?

What if a Wiffle ball but heavy? That one toy every kid had to have. A flying saucer.

What does it look like? A slider going slower, with more frisbee-like horizontal break. Where a typical slider might move 5 inches from arm to glove side and zip in at 85 mph, on sweepers, pitchers are going for 15 inches or more at about 82 mph.

[Read more: How the sweeper became the newest weapon for MLB pitchers]

Why do pitchers throw it? Much like cutters help neutralize opposite-handed batters, baseball’s trendiest recent addition is better against same-side batters — righty vs. righty or lefty vs. lefty. It also offers the sinker-style weak contact associated with horizontal movement, especially helpful against batters who are less prone to striking out. As more pitchers have gotten more granular about their arsenals, they have defined more distinct breaking balls, and the sweeper is a product of that. Learning it often revolves around changing the orientation of the ball in the hand, drawing on lessons the industry has learned about how seams affect movement in flight.

Who has a good one? Shohei Ohtani has fallen in love with his sweeper, throwing it more than any other pitch in 2023. A bushel of prominent relievers, from Evan Phillips to Jason Adam, throw very good ones. Minnesota Twins starter Sonny Gray might be the model for the movement toward multiple, varied breaking ball options. This one slots between his harder cutter and more vertically oriented curveball, and good luck figuring out which one he’s going to throw when.

What is a curveball?

Uncle Charlie. English-language shorthand for the unexpected. The hammer.

What does it look like? To the ubiquitous center-field camera, it looks like a parabola, a pitch with a hump in the middle, the idiom “what goes up must come down” demonstrated on fast-forward. To the batter, it looks like a fastball heading for their face until it parachutes in at their knees. Average velocity: 79.7 mph. With shapes often relayed in relation to a clock — 12 to 6 would be a purely vertical break — the curveball is thrown with topspin, the opposite of a four-seam fastball.

Why do pitchers throw it? The slowest of baseball’s staples, a curveball combines a dramatic change of speed with a huge, looping trajectory. Because of their evil-twin spin relationship, a curve and a four-seam fastball can be indistinguishable in the split second after release.

Who has a good one? Kershaw’s is an institution. Burnes’ is a yakker. The Atlanta Braves’ Max Fried throws hitters for a loop with an almost 20 mph velocity gap between fastball and curve. And young Cleveland Guardians starter Triston McKenzie might have the next great one.

Hybrid pitches — and how fantasy turns to reality

Despite the thorough cataloging of pitch types, some major-leaguers still send baseballs on novel flights. Among those genre-busting or -blending pitches: the “slambio,” the “splinker” and the “churve,” which are 100 percent real, even if they sound like a bad joke about “The Lion, the Witch and the Wardrobe.”

The slambio is New York Yankees reliever Ian Hamilton’s slider-change mashup (cambio is the Spanish term for a changeup). The splinker is Minnesota Twins reliever Jhoan Duran’s ridiculously hard sinker-splitter that zips in around 98 mph. And the churve is New York Mets starter Joey Lucchesi’s oddly gripped change-curveball.

Then, of course, there’s the longstanding slurve, a combination of slider and curve that has largely been roped into the sweeper category. Still, a few pitchers throw pitches that are slurve-y but not sweepers. The Los Angeles Dodgers’ Julio Urias is the prime example, using a whooshing breaking ball that typically doesn’t touch 80 mph, putting it in the curveball velocity band but with sweeper movement.

If this all sounds silly — here today, gone tomorrow, fringe stuff — consider the first public descriptions of the slider, documented by longtime baseball writer Rob Neyer.

“[George] Blaeholder's strong point is his fast ball,” Baseball Magazine wrote in 1936. “He generally throws this with a side-arm motion which gives the ball a curious sweep to one side as it crosses the plate.”

Hitters eventually dubbed that pitch, which was probably a cutter, the “slide ball.” A few years later, another pitcher, George Uhle, rechristened his “sailing fastball” a slider. Indeed, our first instinct is to explain a new thing in terms of the things we already know.

Zombro’s work with pitchers at Tread supports the idea of pitch classifications as an organic process, not a rigid gospel, even as we maintain a useful, public-facing handbook. He said many major-leaguers are capable of creating a sort of rainbow of pitch options by manipulating their arsenals on the fly — an improvisational skill in the vein of Rich Hill’s daring arm angles.

“I think a lot more people than what we probably think are actually able to manipulate the ball all across that continuum,” Zombro said of the operational space between two-seamers and sweepers. “But the ability to scale those pitches within themselves is super, super important because that then essentially provides context to make each pitch multiple in and of themselves.”

Keep that in mind as we move on to the offspeed family of pitches.

What is a changeup?

Bugs Bunny. Slamming on the brakes. A pull of the string.

What does it look like? A sinker but 8-10 mph slower, in the 85 mph range on average. There are several ways to throw a changeup — not even counting the separately categorized splitter — but they all generally put an emphasis on generating arm-side movement. One version, most famously used by Pedro Martinez, is the circle change that ratchets up deception and dials down velocity by putting the pressure of releasing the ball on weaker fingers. Compared to the splitter, changeups drop less and spin more.

Why do pitchers throw it? To blow up a hitter’s timing. Commonly cited as a hurdle young pitchers must clear to establish themselves as starting-pitcher material, the changeup is a weapon used to defang the more threatening bats of opposite-handed hitters.

Who has a good one? Unsurprisingly, you’ll find some of the best changeups coming from pitchers who use sinkers, such as the Marlins’ Alcantara. One rare breed of pitcher is the starter who uses the change to cover for the lack of a trusted breaking ball, e.g. Tyler Anderson. The Tampa Bay Rays’ Shane McClanahan — in a show of how blurry some classification lines can become — throws a fantastic changeup that borders on a splitter. The most eye-popping changeup, though, belongs to Milwaukee Brewers closer Devin Williams and has its own nickname: the Airbender.

What is a splitter?

2 legit 2 hit. A drone crash.

What does it look like? A fastball that drops an engine midway to the plate. In reality, it’s a particularly deceptive changeup, a touch faster at an average of 87 mph, achieved by squeezing the ball between the index and middle fingers. Built to present as a fastball upon release, the splitter tends to fall out of the sky in dramatic, unpredictable and unhittable ways due to a lack of spin.

[Read more: Why splitters are returning to MLB after decades of injury taboo]

Why do pitchers throw it? Because those who can locate a comfortable, consistent grip reap serious rewards. No pitch type earns better results in MLB, though the pitch is still relatively rare in the U.S. due to anecdotal injury worries.

Who has a good one? In Japan, there is no such block about the splitter. Shohei Ohtani and Yu Darvish have thrown the pitch throughout their careers. New York Mets rookie Kodai Senga’s “ghost fork” is a form of splitter. The Texas Rangers’ Nathan Eovaldi is thriving with his, as is Baltimore Orioles closer Felix Bautista. But no one makes more use of the pitch than the San Francisco Giants’ Alex Cobb and the Toronto Blue Jays’ Kevin Gausman.

What is a screwball?

A glitch in the matrix.

What does it look like? If you couldn’t see the pitcher, you’d assume it was a curveball. Since you can see the pitcher and, therefore, realize he throws with the wrong hand to produce a curveball that goes that direction, it looks like the laws of physics stepped out for a coffee break.

Why do pitchers throw it? Weird pitches are often good pitches because hitters have to perform their craft mostly on muscle memory and reaction time. Alas, almost no one throws the screwball because it’s difficult to do.

Who has a good one? There’s only one pitcher in the majors with a screwball: San Diego Padres reliever Brent Honeywell. It’s not the main pitch in his arsenal, but he gets some quizzical looks from hitters when he whips it out.

What is a knuckleball?

Float like a butterfly, sting like a bee.

What does it look like? A feather riding every ripple of wind toward home plate. Using a knuckleball is an exceedingly difficult craft that requires reducing the ball’s spin to virtually zero so that it dances on the air en route to home.

Why do pitchers throw it? Often because they’ve tried every conventional pitch and determined that this is their best shot.

Who has a good one? Nobody in the majors at the moment. The most recent star-level example was R.A. Dickey, who won the 2012 NL Cy Young Award in a fit of glory at age 37, while the Boston Red Sox employed both Tim Wakefield and Steven Wright, two of the longer-running knucklers in recent memory. The last knuckleball thrown in the majors by an actual pitcher — as opposed to a position player goofing off in garbage time — came from Mickey Jannis in a cup of coffee with the Orioles in 2021.

What’s next in pitching evolution?

Believe it or not, this is the tip of the pitching iceberg.

Talk of pitch design and pitch types has crossed over from clubhouses into broadcasts and social media as players have publicly touted the results, but there is much more to it. Still, fans don’t need to have a pulse on all the latest injury-prevention research or the biomechanics of pitching to appreciate an expansion of the sport’s universe.

We are learning more about why certain pitches — and pitchers — succeed. We are gaining terminology for new qualities that might (or might not) unlock the next ace. And to the extent that the difference-making worlds of player development and pitching strategy can become more visible, more comprehensible as part of the game’s entertainment value through classification and the resulting curiosity, that’s a plus.

“I don't know that having 40 classifications would be a good idea,” Langin said. “But it's cool to see that that side of the game, the development side, has gotten the interest it has, to where guys are talking that much about it.”

Thanks to Robert Au of Baseball Prospectus for research assistance.

Zach CrizerMLB writer

Zach Crizer is a writer based in New York. He's an author for Baseball Prospectus, and his work has also been featured on MLB.com.