Glossary

The stuff grade. Pitch quality estimated from physical characteristics alone — velocity, movement, and time to plate — with no outcomes, counts, location, or pitcher identity. Expressed as a z-score within season × pitch type: 0 is a league-average pitch of that type, +1 is one standard deviation above. Higher is better for the pitcher. The scale is anchored to books of 50+ pitches; smaller books are scored on that same scale.

The velocity greek: the change in zStuff per +1 mph. Priced along the empirical path a real velocity change takes — the movement and flight-time changes that typically ride along with a tick are included, at league-wide within-pitcher rates. The slope of the "Stuff vs Velocity" curve at the pitcher's operating point.

The change in Delta per +1 mph — the curvature of stuff in velocity. Positive gamma means each additional tick is worth more than the last; the hardest throwers tend to carry the largest deltas because of it.

The ride greek: the change in zStuff per +1 inch of induced vertical break, with typical ride-along changes in the other traits included. The slope of the "Stuff vs IVB" curve at the pitcher's operating point.

The horizontal-movement greek: the change in zStuff per +1 inch of horizontal break, ride-alongs included. Signed by the HB convention below — a positive Vega-X means more cut helps; a negative one means more run helps.