Classifying pitches is more of an art form than a hard science. Each pitcher has a slightly different grip and arm action for their pitches, so the same pitches can technically look quite different depending on the pitcher. Roy Halladay’s two-seam fastball may move in a particular way in relation to his four-seam fastball, but that doesn’t mean all pitchers have similar movement patterns; every pitcher is unique.
That said, pitches tend to move in the same general way in comparison with one another, so it’s relatively easy to take a glance at a pitcher’s movement chart and tell what sort of pitches they throw. Here’s a sample movement chart for a right-handed pitcher (for left-handed pitchers, just imagine the mirror image of this chart).
Note: these charts are presented from the catcher’s point of view. Also, pitches obviously don’t break “up” — gravity pulls every pitch down as it approaches the plate. These movement plots are based around a hypothetical pitch that has absolutely no spin, so when a pitch breaks “up”, it means that it does not fall as much on the way to the plate as a spin-neutral pitch would.
FA = fastball
FF = four-seam fastball
FT = two-seam fastball
FC = fastball (cutter)
FS / SI / SF = fastball (sinker, split-fingered)
You’ll see fastballs classified all number of ways, considering that pitchers throw a wide variety of pitches that are all 90-ish MPH and have similar movements. Four-seam fastballs are considered the standard, and you’ll see those either abbreviated as FF or FA. Two-seam fastballs are the other main type of fastball, and you’ll normally see those abbreviated as FT — although at Brooks Baseball they’re listed as sinkers (SI) since they tend to act in a similar manner.
When you the abbreviation FA in conjunction with FT, it represents four-seamers. But when you see FA as a stand-alone category and no other fastball type listed, it’s being used as a catch-all category that includes both four-seamers and two-seamers.
Cutters are essentially fast sliders that don’t break quite as much. And sinkers, splitters, etc. have similar horizontal movement to four-seam fastballs but don’t have as much rise.
SL = slider
CH = changeup
CB / CU = curveball
KC = knuckle-curve
KN = knuckleball
EP = eephus
These are all relatively straightforward. Changeups have similar movement to fastballs, but they tend to sink a little bit more than four-seamers and (obviously) they travel considerably slower. Curveballs have the most break of any off-speed pitch, and sliders also have a large break and travel faster.
You may see some pitches classified as knuckle-curves, but they’ll be difficult to distinguish from regular curveballs on pitch movement charts. Curves and knuckle-curves generally move the same; the main difference is in how pitchers grip and throw them.
UN / XX = unidentified
PO / FO = pitch out