How muscle fiber direction affects perceived tenderness
Muscle fibers run in a single direction within each muscle bundle. Cutting across the grain shortens the fiber length exposed in each bite, making the meat feel more tender by reducing the chewing effort required. Cutting parallel to the grain (with the grain) leaves long intact fibers that feel tougher and require more chewing.
This effect is purely mechanical: cutting across the grain doesn't change the fiber's toughness, but it reduces the length the jaw must separate and compress. A tough cut like brisket that has been braised remains relatively less tender when sliced with the grain compared to the same braised brisket sliced against the grain.
Identifying grain direction in different muscles
The grain runs lengthwise along each muscle because fibers are pulled in one direction by their attachment points. In a steak or roast, the grain direction becomes visible when light reflects off the exposed muscle fibers after cutting. Run your knife perpendicular to this visible direction.
Different muscles have different grain patterns. The muscle fibers in a flat iron steak run horizontally across the meat, so diagonal cuts across that direction shorten fibers more effectively. Flank steak fibers run at an angle, so cutting at 45 degrees perpendicular to the surface is optimal. Slower-cooked cuts like pot roast become tender regardless of cutting direction because collagen converts to gelatin, but cutting across the grain still enhances the texture.