What separates “good” bread from mediocre? Some loaves (a sourdough boule, for example) should have a crackly crust — the kind that will scatter geometric crumbs across your cutting board as you slice into it — surrounding a slightly airy, well-hydrated crumb. Others (such as rectangular loaves of rye) fall in a small but mighty category: They’re dense, barely leavened and full of whole grains, the crust more chewy than snappy. In all cases, there should be a definite character to the flavor, due to the various wheat, grains and seeds in the mix.