Hollow-Ground

A hollow-ground bevel is sharpened with a concave curve rather than a flat plane, produced by grinding against a wheel whose curved surface removes more material just behind the edge than right at it. The thinner cross-section directly behind a hollow-ground edge means less steel dragging through the cut, which is why hollow-ground blades -- table saw blades marketed for glue-line ripping, some hand plane irons -- are prized for an exceptionally smooth, low-friction cut.

The tradeoff is edge durability: a hollow-ground edge is inherently thinner and more delicate right where it matters most, and tends to dull or chip faster under hard use than a flat-ground edge with more steel supporting it. It's a specialty grind for a smoother cut, not a general-purpose upgrade.