The performance difference for a hitter or pitcher depending on the handedness of the opponent.
Most hitters perform better against opposite-handed pitching, and many pitchers are more effective against same-handed batters. Platoon splits quantify that gap — for example, a right-handed hitter's numbers against left-handed pitching versus right-handed pitching.
Splits can be noisy in small samples, especially the short side of a platoon, so they are best weighted by sample size and combined with the underlying skills rather than read in isolation.
Handedness is one of the first filters in any matchup: a hitter's vs-hand split against tonight's starter sets the baseline before contact-quality and park factors refine it.
They capture a real, persistent advantage — most hitters hit opposite-handed pitching better. A favorable handedness matchup raises the baseline for a hitter's props before any other factor is applied.