Hi Karl, a preferment is a type of starter that is composed of a portion of the flour and water from a recipe, plus a little yeast or sourdough starter. Typically this starter is allowed to rise overnight or for 12-16 hours and then added in with the remaining ingredients when the dough is mixed. A sourdough preferment is often called a "levain." A typical preferment you'll see in baguette recipes is called a "poolish" and is composed of equal parts flour and water by weight, along with just a little yeast. Pre-fermenting a portion of the flour in a recipe results in deeper flavor development, increases the strength of the dough, and also improves the keeping quality of the bread. Barb

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