The main difference between bread flour and all-purpose flour is a matter of protein. Bread flour, which comes in white and whole wheat varieties, has a higher protein content than all-purpose, usually 11-13%. It’s called “bread flour” because most bread requires higher amounts of protein to produce lots of gluten. Gluten is the stringy strands that give bread dough its stretch and elasticity, and baked bread its characteristic chew. Kneading dough develops a network of gluten strands that trap air and produce the airy holes characteristic of many breads. You can use bread flour in place of AP flour when you actually want a chewier result—in pizza dough, for instance—but you don't want to use it in place of cake or pastry flour, or in any baked goods that you want to be light and tender.