That’s the obvious question, isn’t it? And indeed one that several of you have asked. Most breads, after all, call for only one kind of sponge or starter. So what gives adding two of them? The answer is that different types of microbes do better in different environments. Some prefer it very wet (poolish) some prefer it a little drier (biga), some even drier still (old dough). By combining preferments in the same dough, you introduce a wider variety of flavor-creating bugs, and by extension, create a wider variety of flavors.