The pattern of air pockets (alveoli) inside your bread, determined by fermentation, shaping, and baking.
An open crumb has large, irregular holes and is prized for artisan bread. A tight crumb has small, uniform holes and is better for sandwiches. You can influence crumb structure through hydration (more water = more open), fermentation length (longer = more open, up to a point), and shaping technique (tighter shaping = more even crumb). Stretch and folds during bulk fermentation help redistribute gas for a more uniform crumb.
